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Introduction

This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript
(Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0
browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet
Explorer 3.0.

The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma
General Assembly of June 1997.

That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international
standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it
fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.

The third edition of the Standard introduced powerful regular expressions, better string handling, new control statements,
try/catch exception handling, tighter definition of errors, formatting for numeric output and minor changes in anticipation of
forthcoming internationalisation facilities and future language growth. The third edition of the ECMAScript standard was adopted
by the Ecma General Assembly of December 1999 and published as ISO/IEC 16262:2002 in June 2002.

Since publication of the third edition, ECMAScript has achieved massive adoption in conjunction with the World Wide Web where
it has become the programming language that is supported by essentially all web browsers. Significant work was done to develop a
fourth edition of ECMAScript. Although that work was not completed and not published as the fourth edition of ECMAScript, it
informs continuing evolution of the language. The fifth edition of ECMAScript (published as ECMA-262 5th edition)
codifies de facto interpretations of the language specification that have become common among browser implementations and adds
support for new features that have emerged since the publication of the third edition. Such features include accessor
properties, reflective creation and inspection of objects, program control of property attributes, additional array manipulation
functions, support for the JSON object encoding format, and a strict mode that provides enhanced error checking and program
security.

This present edition 5.1 of the ECMAScript Standard is fully aligned with third edition of the international standard ISO/IEC
16262:2011.

ECMAScript is a vibrant language and the evolution of the language is not complete. Significant technical enhancement will
continue with future editions of this specification.

This Ecma Standard has been adopted by the General Assembly of June 2011.

A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties, functions, and
program syntax and semantics described in this specification.

A conforming implementation of this Standard shall interpret characters in conformance with the Unicode Standard, Version 3.0
or later and ISO/IEC 10646-1 with either UCS-2 or UTF-16 as the adopted encoding form, implementation level 3. If the adopted
ISO/IEC 10646-1 subset is not otherwise specified, it is presumed to be the BMP subset, collection 300. If the adopted encoding
form is not otherwise specified, it presumed to be the UTF-16 encoding form.

A conforming implementation of ECMAScript is permitted to provide additional types, values, objects, properties, and
functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript is permitted to
provide properties not described in this specification, and values for those properties, for objects that are described in this
specification.

A conforming implementation of ECMAScript is permitted to support program and regular expression syntax not described in this
specification. In particular, a conforming implementation of ECMAScript is permitted to support program syntax that makes use of
the “future reserved words” listed in 7.6.1.2 of this specification.

The following referenced documents are indispensable for the application of this document. For dated references, only the
edition cited applies. For undated references, the latest edition of the referenced document (including any amendments)
applies.

This section contains a non-normative overview of the ECMAScript language.

ECMAScript is an object-oriented programming language for performing computations and manipulating computational objects
within a host environment. ECMAScript as defined here is not intended to be computationally self-sufficient; indeed, there are
no provisions in this specification for input of external data or output of computed results. Instead, it is expected that the
computational environment of an ECMAScript program will provide not only the objects and other facilities described in this
specification but also certain environment-specific host objects, whose description and behaviour are beyond the scope of
this specification except to indicate that they may provide certain properties that can be accessed and certain functions that
can be called from an ECMAScript program.

A scripting language is a programming language that is used to manipulate, customise, and automate the
facilities of an existing system. In such systems, useful functionality is already available through a user interface, and the
scripting language is a mechanism for exposing that functionality to program control. In this way, the existing system is said
to provide a host environment of objects and facilities, which completes the capabilities of the scripting language. A scripting
language is intended for use by both professional and non-professional programmers.

ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven Web pages
in browsers and to perform server computation as part of a Web-based client-server architecture. ECMAScript can provide core
scripting capabilities for a variety of host environments, and therefore the core scripting language is specified in this
document apart from any particular host environment.

Some of the facilities of ECMAScript are similar to those used in other programming languages; in particular Java™,
Self, and Scheme as described in:

A web browser provides an ECMAScript host environment for client-side computation including, for instance, objects that
represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames, history, cookies, and input/output. Further, the
host environment provides a means to attach scripting code to events such as change of focus, page and image loading,
unloading, error and abort, selection, form submission, and mouse actions. Scripting code appears within the HTML and the
displayed page is a combination of user interface elements and fixed and computed text and images. The scripting code is
reactive to user interaction and there is no need for a main program.

A web server provides a different host environment for server-side computation including objects representing requests,
clients, and files; and mechanisms to lock and share data. By using browser-side and server-side scripting together, it is
possible to distribute computation between the client and server while providing a customised user interface for a Web-based
application.

Each Web browser and server that supports ECMAScript supplies its own host environment, completing the ECMAScript execution
environment.

The following is an informal overview of ECMAScript—not all parts of the language are described. This overview is not
part of the standard proper.

ECMAScript is object-based: basic language and host facilities are provided by objects, and an ECMAScript program is a
cluster of communicating objects. An ECMAScript object is a collection of properties each with
zero or more attributes that determine how each property can be used—for example, when the Writable
attribute for a property is set to false, any attempt by executed ECMAScript code to change the value of the property
fails. Properties are containers that hold other objects, primitive values, or functions. A
primitive value is a member of one of the following built-in types: Undefined, Null, Boolean,
Number, and String; an object is a member of the remaining built-in type Object; and a function is a
callable object. A function that is associated with an object via a property is a method.

ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript entities.
These built-in objects include the global object, the Object object, the Function object, the Array
object, the String object, the Boolean object, the Number object, the Math object, the Date
object, the RegExp object, the JSON object, and the Error objects Error, EvalError, RangeError,
ReferenceError, SyntaxError, TypeError and URIError.

ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as an easy-to-use
scripting language. For example, a variable is not required to have its type declared nor are types associated with
properties, and defined functions are not required to have their declarations appear textually before calls to them.

ECMAScript does not use classes such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways
including via a literal notation or via constructors which create objects and then execute code that
initialises all or part of them by assigning initial values to their properties. Each constructor is a function that has a
property named “prototype” that is used to implement prototype-based inheritance and
shared properties. Objects are created by using constructors in new expressions; for example, new
Date(2009,11) creates a new Date object. Invoking a constructor without using new has consequences that depend
on the constructor. For example, Date() produces a string representation of the current date and time rather
than an object.

Every object created by a constructor has an implicit reference (called the object’s prototype) to the value
of its constructor’s “prototype” property. Furthermore, a prototype may have a non-null
implicit reference to its prototype, and so on; this is called the prototype chain. When a reference is made to a
property in an object, that reference is to the property of that name in the first object in the prototype chain that
contains a property of that name. In other words, first the object mentioned directly is examined for such a property; if
that object contains the named property, that is the property to which the reference refers; if that object does not contain
the named property, the prototype for that object is examined next; and so on.

Figure 1 — Object/Prototype Relationships

In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and
inheritance is only of structure and behaviour. In ECMAScript, the state and methods are carried by objects, and structure,
behaviour, and state are all inherited.

All objects that do not directly contain a particular property that their prototype contains share that property and its
value. Figure 1 illustrates this:

CF is a constructor (and also an object). Five objects have been created by using new expressions:
cf1, cf2, cf3, cf4, and cf5. Each
of these objects contains properties named q1 and q2. The dashed lines represent the implicit prototype relationship; so, for example,
cf3’s prototype is CFp. The constructor, CF, has two properties itself,
named P1 and P2, which are not
visible to CFp, cf1, cf2, cf3,
cf4, or cf5. The property named CFP1 in
CFp is shared by cf1, cf2, cf3,
cf4, and cf5 (but not by CF), as are any properties found in
CFp’s implicit prototype chain that are not named q1,
q2, or CFP1. Notice that there is no
implicit prototype link between CF and CFp.

Unlike class-based object languages, properties can be added to objects dynamically by assigning values to them. That is,
constructors are not required to name or assign values to all or any of the constructed object’s properties. In the
above diagram, one could add a new shared property for cf1, cf2, cf3,
cf4, and cf5by assigning a new value to the property in CFp.

The ECMAScript Language recognises the possibility that some users of the language may wish to restrict their usage of
some features available in the language. They might do so in the interests of security, to avoid what they consider to be
error-prone features, to get enhanced error checking, or for other reasons of their choosing. In support of this
possibility, ECMAScript defines a strict variant of the language. The strict variant of the language excludes some specific
syntactic and semantic features of the regular ECMAScript language and modifies the detailed semantics of some features. The
strict variant also specifies additional error conditions that must be reported by throwing error exceptions in situations
that are not specified as errors by the non-strict form of the language.

The strict variant of ECMAScript is commonly referred to as the strict mode of the language. Strict mode selection
and use of the strict mode syntax and semantics of ECMAScript is explicitly made at the level of individual ECMAScript code
units. Because strict mode is selected at the level of a syntactic code unit, strict mode only imposes restrictions that
have local effect within such a code unit. Strict mode does not restrict or modify any aspect of the ECMAScript semantics
that must operate consistently across multiple code units. A complete ECMAScript program may be composed for both strict
mode and non-strict mode ECMAScript code units. In this case, strict mode only applies when actually executing code that is
defined within a strict mode code unit.

In order to conform to this specification, an ECMAScript implementation must implement both the full unrestricted
ECMAScript language and the strict mode variant of the ECMAScript language as defined by this specification. In addition, an
implementation must support the combination of unrestricted and strict mode code units into a
single composite program.

NOTE When a constructor creates an object, that object implicitly references the
constructor’s “prototype” property for the purpose of resolving property references. The
constructor’s “prototype” property can be referenced by the program expression constructor.prototype, and properties added to an object’s
prototype are shared, through inheritance, by all objects sharing the prototype. Alternatively, a new object may be
created with an explicitly specified prototype by using the Object.create built-in function.

object supplied by an ECMAScript implementation, independent of the host environment, that is present at the start of the
execution of an ECMAScript program

NOTE Standard built-in objects are defined in this specification, and an ECMAScript
implementation may specify and define others. Every built-in object is a native object. A built-in constructor is a
built-in object that is also a constructor.

member of the Object type that is an instance of the standard built-in Boolean constructor

NOTE A Boolean object is created by using the Boolean constructor in a
new expression, supplying a Boolean value as an argument. The resulting object has an internal property whose
value is the Boolean value. A Boolean object can be coerced to a Boolean value.

primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer

NOTE A String value is a member of the String type. Each integer value in the sequence usually
represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or requirements on the
values except that they must be 16-bit unsigned integers.

member of the Object type that is an instance of the standard built-in String constructor

NOTE A String object is created by using the String constructor in a
new expression, supplying a String value as an argument. The resulting object has an internal property whose
value is the String value. A String object can be coerced to a String value by calling the String constructor
as a function (15.5.1).

member of the Object type that is an instance of the standard built-in Number constructor

NOTE A Number object is created by using the Number constructor in a
new expression, supplying a Number value as an argument. The resulting object has an internal property whose
value is the Number value. A Number object can be coerced to a Number value by calling the Number constructor
as a function (15.7.1).

NOTE Depending upon the form of the property the value may be represented either directly as a
data value (a primitive value, an object, or a function object) or indirectly by a pair of accessor functions.

A context-free grammar consists of a number of productions. Each production has an abstract symbol called a
nonterminal as its left-hand side, and a sequence of zero or more nonterminal and terminal symbols as
its right-hand side. For each grammar, the terminal symbols are drawn from a specified alphabet.

Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given
context-free grammar specifies a language, namely, the (perhaps infinite) set of possible sequences of terminal
symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for
which the nonterminal is the left-hand side.

A lexical grammar for ECMAScript is given in clause 7. This grammar has as its terminal
symbols characters (Unicode code units) that conform to the rules for SourceCharacter defined in Clause 6. It defines a set of productions, starting from the goal symbol InputElementDiv or InputElementRegExp, that describe how sequences of such
characters are translated into a sequence of input elements.

Input elements other than white space and comments form the terminal symbols for the syntactic grammar for ECMAScript and
are called ECMAScript tokens. These tokens are the reserved words, identifiers, literals, and punctuators of the
ECMAScript language. Moreover, line terminators, although not considered to be tokens, also become part of the stream of
input elements and guide the process of automatic semicolon insertion (7.9). Simple white space and
single-line comments are discarded and do not appear in the stream of input elements for the syntactic grammar. A MultiLineComment (that is, a comment of the form “/*…*/” regardless of whether it spans more than
one line) is likewise simply discarded if it contains no line terminator; but if a MultiLineComment
contains one or more line terminators, then it is replaced by a single line terminator, which becomes part of the stream of
input elements for the syntactic grammar.

A RegExp grammar for ECMAScript is given in 15.10. This grammar also has as its terminal
symbols the characters as defined by SourceCharacter. It defines a set of productions, starting from
the goal symbol Pattern, that describe how sequences of characters are translated into regular
expression patterns.

Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as
separating punctuation. The lexical and RegExp grammars share some productions.

Another grammar is used for translating Strings into numeric values. This grammar is similar to the part of the lexical
grammar having to do with numeric literals and has as its terminal symbols SourceCharacter. This
grammar appears in 9.3.1.

Productions of the numeric string grammar are distinguished by having three colons “:::” as
punctuation.

The syntactic grammar for ECMAScript is given in clauses 11, 12, 13 and 14. This grammar has ECMAScript tokens
defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set of productions,
starting from the goal symbol Program, that describe how sequences of tokens can form syntactically
correct ECMAScript programs.

When a stream of characters is to be parsed as an ECMAScript program, it is first converted to a stream of input elements
by repeated application of the lexical grammar; this stream of input elements is then parsed by a single application of the
syntactic grammar. The program is syntactically in error if the tokens in the stream of input elements cannot be parsed as a
single instance of the goal nonterminal Program, with no tokens left over.

Productions of the syntactic grammar are distinguished by having just one colon “:” as
punctuation.

The syntactic grammar as presented in clauses 11, 12, 13 and 14 is actually not a complete account of which token
sequences are accepted as correct ECMAScript programs. Certain additional token sequences are also accepted, namely, those
that would be described by the grammar if only semicolons were added to the sequence in certain places (such as before line
terminator characters). Furthermore, certain token sequences that are described by the grammar are not considered acceptable
if a terminator character appears in certain “awkward” places.

The JSON grammar is used to translate a String describing a set of ECMAScript objects into actual objects. The JSON
grammar is given in 15.12.1.

The JSON grammar consists of the JSON lexical grammar and the JSON syntactic grammar. The JSON lexical grammar is used to
translate character sequences into tokens and is similar to parts of the ECMAScript lexical grammar. The JSON syntactic
grammar describes how sequences of tokens from the JSON lexical grammar can form syntactically correct JSON object
descriptions.

Productions of the JSON lexical grammar are distinguished by having two colons “::” as separating
punctuation. The JSON lexical grammar uses some productions from the ECMAScript lexical grammar. The JSON syntactic grammar
is similar to parts of the ECMAScript syntactic grammar. Productions of the JSON syntactic grammar are distinguished by
using one colon “:” as separating punctuation.

Terminal symbols of the lexical, RegExp, and numeric string grammars, and some of the terminal symbols of the other
grammars, are shown in fixed width font, both in the productions of the grammars and throughout this
specification whenever the text directly refers to such a terminal symbol. These are to appear in a program exactly as
written. All terminal symbol characters specified in this way are to be understood as the appropriate Unicode character from
the ASCII range, as opposed to any similar-looking characters from other Unicode ranges.

Nonterminal symbols are shown in italic type. The definition of a nonterminal is introduced by the name of the
nonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar the production
belongs.) One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the
syntactic definition:

WhileStatement:

while(Expression)Statement

states that the nonterminal WhileStatement represents the token while, followed by a
left parenthesis token, followed by an Expression, followed by a right parenthesis token, followed
by a Statement. The occurrences of Expression and Statement are themselves nonterminals. As another example, the syntactic definition:

ArgumentList:

AssignmentExpression

ArgumentList,AssignmentExpression

states that an ArgumentList may represent either a single AssignmentExpression or an ArgumentList, followed by a comma, followed by an AssignmentExpression. This definition of ArgumentList is recursive, that is, it is
defined in terms of itself. The result is that an ArgumentList may contain any positive number of
arguments, separated by commas, where each argument expression is an AssignmentExpression. Such
recursive definitions of nonterminals are common.

The subscripted suffix “opt”, which may appear after a terminal or nonterminal, indicates an
optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides, one that omits the
optional element and one that includes it. This means that:

VariableDeclaration:

IdentifierInitialiseropt

is a convenient abbreviation for:

VariableDeclaration:

Identifier

IdentifierInitialiser

and that:

IterationStatement:

for(ExpressionNoInopt;Expressionopt;Expressionopt)Statement

is a convenient abbreviation for:

IterationStatement:

for(;Expressionopt;Expressionopt)Statement

for(ExpressionNoIn;Expressionopt;Expressionopt)Statement

which in turn is an abbreviation for:

IterationStatement:

for(;;Expressionopt)Statement

for(;Expression;Expressionopt)Statement

for(ExpressionNoIn;;Expressionopt)Statement

for(ExpressionNoIn;Expression;Expressionopt)Statement

which in turn is an abbreviation for:

IterationStatement:

for(;;)Statement

for(;;Expression)Statement

for(;Expression;)Statement

for(;Expression;Expression)Statement

for(ExpressionNoIn;;)Statement

for(ExpressionNoIn;;Expression)Statement

for(ExpressionNoIn;Expression;)Statement

for(ExpressionNoIn;Expression;Expression)Statement

so the nonterminal IterationStatement actually has eight alternative right-hand sides.

When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the
terminal symbols on the following line or lines is an alternative definition. For example, the lexical grammar for
ECMAScript contains the production:

NonZeroDigit::one of

123456789

which is merely a convenient abbreviation for:

NonZeroDigit::

1

2

3

4

5

6

7

8

9

If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the production's
right-hand side contains no terminals or nonterminals.

If the phrase “[lookahead ∉ set]” appears in the right-hand side of a production, it
indicates that the production may not be used if the immediately following input token is a member of the given
set. The set can be written as a list of terminals enclosed in curly braces. For convenience, the set
can also be written as a nonterminal, in which case it represents the set of all terminals to which that nonterminal could
expand. For example, given the definitions

DecimalDigit::one of

0123456789

DecimalDigits::

DecimalDigit

DecimalDigitsDecimalDigit

the definition

LookaheadExample::

n[lookahead ∉ {1, 3, 5, 7, 9}]DecimalDigits

DecimalDigit[lookahead ∉ DecimalDigit]

matches either the letter n followed by one or more decimal digits the first of which is even, or a decimal
digit not followed by another decimal digit.

If the phrase “[no LineTerminator here]” appears in the right-hand side of a
production of the syntactic grammar, it indicates that the production is a restricted production: it may not be used
if a LineTerminator occurs in the input stream at the indicated position. For example, the
production:

ThrowStatement:

throw[no LineTerminator here]Expression;

indicates that the production may not be used if a LineTerminator occurs in the program between
the throw token and the Expression.

Unless the presence of a LineTerminator is forbidden by a restricted production, any number of
occurrences of LineTerminator may appear between any two consecutive tokens in the stream of input
elements without affecting the syntactic acceptability of the program.

When an alternative in a production of the lexical grammar or the numeric string grammar appears to be a multi-character
token, it represents the sequence of characters that would make up such a token.

The right-hand side of a production may specify that certain expansions are not permitted by using the phrase
“but not” and then indicating the expansions to be excluded. For example, the production:

Identifier::

IdentifierNamebut notReservedWord

means that the nonterminal Identifier may be replaced by any sequence of characters that could
replace IdentifierName provided that the same sequence of characters could not replace ReservedWord.

Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases where it would be
impractical to list all the alternatives:

The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used to precisely
specify the required semantics of ECMAScript language constructs. The algorithms are not intended to imply the use of any
specific implementation technique. In practice, there may be more efficient algorithms available to implement a given
feature.

In order to facilitate their use in multiple parts of this specification, some algorithms, called abstractoperations, are named and written in parameterised functional form so that they may be referenced by name from within
other algorithms.

When an algorithm is to produce a value as a result, the directive “return
x” is used to indicate that the result of the algorithm is the value of x and that the
algorithm should terminate. The notation Result(n) is used as
shorthand for “the result of step n”.

For clarity of expression, algorithm steps may be subdivided into sequential substeps. Substeps are indented and may
themselves be further divided into indented substeps. Outline numbering conventions are used to identify substeps with the
first level of substeps labelled with lower case alphabetic characters and the second level of substeps labelled with lower
case roman numerals. If more than three levels are required these rules repeat with the fourth level using numeric labels.
For example:

Top-level step

Substep.

Substep

Subsubstep.

Subsubstep.

Subsubsubstep

Subsubsubsubstep

A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the substeps
are only applied if the predicate is true. If a step or substep begins with the word “else”, it is a predicate
that is the negation of the preceding “if” predicate step at the same level.

A step may specify the iterative application of its substeps.

A step may assert an invariant condition of its algorithm. Such assertions are used to make explicit algorithmic
invariants that would otherwise be implicit. Such assertions add no additional semantic requirements and hence need not be
checked by an implementation. They are used simply to clarify algorithms.

Mathematical operations such as addition, subtraction, negation, multiplication, division, and the mathematical functions
defined later in this clause should always be understood as computing exact mathematical results on mathematical real numbers,
which do not include infinities and do not include a negative zero that is distinguished from positive zero. Algorithms in
this standard that model floating-point arithmetic include explicit steps, where necessary, to handle infinities and signed
zero and to perform rounding. If a mathematical operation or function is applied to a floating-point number, it should be
understood as being applied to the exact mathematical value represented by that floating-point number; such a floating-point
number must be finite, and if it is +0 or −0 then the
corresponding mathematical value is simply 0.

The mathematical function abs(x) yields the absolute value of
x, which is −x if x is negative (less
than zero) and otherwise is x itself.

The mathematical function sign(x) yields 1 if x is positive and −1 if
x is negative. The sign function is not used in this standard for cases when x is zero.

The notation “x modulo y” (y must be
finite and nonzero) computes a value k of the same sign as y (or zero) such that abs(k) < abs(y) and x−k = q×y for some integer q.

The mathematical function floor(x) yields the largest integer
(closest to positive infinity) that is not larger than x.

NOTEfloor(x) = x−(x modulo
1).

If an algorithm is defined to “throw an exception”, execution of the algorithm is terminated and no result is
returned. The calling algorithms are also terminated, until an algorithm step is reached that explicitly deals with the
exception, using terminology such as “If an exception was thrown…”. Once such an algorithm step has been
encountered the exception is no longer considered to have occurred.

ECMAScript source text is represented as a sequence of characters in the Unicode character encoding, version 3.0 or later.
The text is expected to have been normalised to Unicode Normalization Form C (canonical composition), as described in Unicode
Technical Report #15. Conforming ECMAScript implementations are not required to perform any normalisation of text, or behave as
though they were performing normalisation of text, themselves. ECMAScript source text is assumed to be a sequence of 16-bit
code units for the purposes of this specification. Such a source text may include sequences of 16-bit code units that are not
valid UTF-16 character encodings. If an actual source text is encoded in a form other than 16-bit code units it must be
processed as if it was first converted to UTF-16.

Syntax

SourceCharacter::

any Unicode code unit

Throughout the rest of this document, the phrase “code unit” and the word “character” will be used to
refer to a 16-bit unsigned value used to represent a single 16-bit unit of text. The phrase “Unicode character” will
be used to refer to the abstract linguistic or typographical unit represented by a single Unicode scalar value (which may be
longer than 16 bits and thus may be represented by more than one code unit). The phrase “code point” refers to such
a Unicode scalar value. “Unicode character” only refers to entities represented by single Unicode scalar values: the
components of a combining character sequence are still individual “Unicode characters,” even though a user might
think of the whole sequence as a single character.

In string literals, regular expression literals, and identifiers, any character (code unit) may also be expressed as a
Unicode escape sequence consisting of six characters, namely \u plus four hexadecimal digits. Within a comment,
such an escape sequence is effectively ignored as part of the comment. Within a string literal or regular expression literal,
the Unicode escape sequence contributes one character to the value of the literal. Within an identifier, the escape sequence
contributes one character to the identifier.

NOTE Although this document sometimes refers to a “transformation” between a
“character” within a “string” and the 16-bit unsigned integer that is the code unit of that character,
there is actually no transformation because a “character” within a “string” is actually represented
using that 16-bit unsigned value.

ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program, if the
Unicode escape sequence \u000A, for example, occurs within a single-line comment, it is interpreted as a line
terminator (Unicode character 000A is line feed) and therefore the next character is not part of the comment.
Similarly, if the Unicode escape sequence \u000A occurs within a string literal in a Java program, it is likewise
interpreted as a line terminator, which is not allowed within a string literal—one must write \n instead of
\u000A to cause a line feed to be part of the string value of a string literal. In an ECMAScript program, a Unicode
escape sequence occurring within a comment is never interpreted and therefore cannot contribute to termination of the comment.
Similarly, a Unicode escape sequence occurring within a string literal in an ECMAScript program always contributes a character
to the String value of the literal and is never interpreted as a line terminator or as a quote mark that might terminate the
string literal.

The source text of an ECMAScript program is first converted into a sequence of input elements, which are tokens, line
terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible
sequence of characters as the next input element.

There are two goal symbols for the lexical grammar. The InputElementDiv symbol is used in those
syntactic grammar contexts where a leading division (/) or division-assignment (/=) operator is
permitted. The InputElementRegExp symbol is used in other syntactic grammar contexts.

NOTE There are no syntactic grammar contexts where both a leading division or division-assignment,
and a leading RegularExpressionLiteral are permitted. This is not affected by semicolon insertion (see 7.9); in examples such as the following:

a = b/hi/g.exec(c).map(d);

where the first non-whitespace, non-comment character after a LineTerminator is slash
(/) and the syntactic context allows division or division-assignment, no semicolon is inserted at the LineTerminator. That is, the above example is interpreted in the same way as:

a = b / hi / g.exec(c).map(d);

Syntax

The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database
such as left-to-right mark or right-to-left mark) are control codes used to control the formatting of a range of text in the
absence of higher-level protocols for this (such as mark-up languages).

It is useful to allow format-control characters in source text to facilitate editing and display. All format control
characters may be used within comments, and within string literals and regular expression literals.

<ZWNJ> and <ZWJ> are format-control characters that are used to make necessary distinctions when forming words or
phrases in certain languages. In ECMAScript source text, <ZWNJ> and
<ZWJ> may also be used in an identifier after the first character.

<BOM> is a format-control character used primarily at the start of
a text to mark it as Unicode and to allow detection of the text's encoding and byte order. <BOM> characters intended for this purpose can sometimes also appear after the start of a text, for
example as a result of concatenating files. <BOM> characters are treated as white space characters (see 7.2).

The special treatment of certain format-control characters outside of comments, string literals, and regular expression
literals is summarised in Table 1.

White space characters are used to improve source text readability and to separate tokens (indivisible lexical units) from
each other, but are otherwise insignificant. White space characters may occur between any two tokens and at the start or end
of input. White space characters may also occur within a StringLiteral or a RegularExpressionLiteral (where they are considered significant characters forming part of the literal
value) or within a Comment, but cannot appear within any other kind of token.

The ECMAScript white space characters are listed in Table 2.

Table 2 — Whitespace Characters

Code Unit Value

Name

Formal Name

\u0009

Tab

<TAB>

\u000B

Vertical Tab

<VT>

\u000C

Form Feed

<FF>

\u0020

Space

<SP>

\u00A0

No-break space

<NBSP>

\uFEFF

Other category “Zs”

Byte Order Mark

Any other Unicode “space separator”

<BOM>

<USP>

ECMAScript implementations must recognise all of the white space characters defined in Unicode 3.0. Later editions of the
Unicode Standard may define other white space characters. ECMAScript implementations may recognise white space characters from
later editions of the Unicode Standard.

Syntax

Like white space characters, line terminator characters are used to improve source text readability and to separate tokens
(indivisible lexical units) from each other. However, unlike white space characters, line terminators have some influence over
the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few
places where they are forbidden by the syntactic grammar. Line terminators also affect the process of automatic semicolon insertion (7.9). A line terminator cannot occur within any token except a StringLiteral. Line terminators may only occur within a StringLiteral token as part
of a LineContinuation.

A line terminator can occur within a MultiLineComment (7.4) but cannot occur
within a SingleLineComment.

Line terminators are included in the set of white space characters that are matched by the \s class in regular
expressions.

The ECMAScript line terminator characters are listed in Table 3.

Table 3 — Line Terminator Characters

Code Unit Value

Name

Formal Name

\u000A

Line Feed

<LF>

\u000D

Carriage Return

<CR>

\u2028

Line separator

<LS>

\u2029

Paragraph separator

<PS>

Only the characters in Table 3 are treated as line terminators. Other new line or line breaking characters are treated as
white space but not as line terminators. The character sequence <CR><LF> is commonly used as a line terminator. It
should be considered a single character for the purpose of reporting line numbers.

Syntax

Comments can be either single or multi-line. Multi-line comments cannot nest.

Because a single-line comment can contain any character except a LineTerminator character, and
because of the general rule that a token is always as long as possible, a single-line comment always consists of all
characters from the // marker to the end of the line. However, the LineTerminator at the
end of the line is not considered to be part of the single-line comment; it is recognised separately by the lexical grammar
and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies
that the presence or absence of single-line comments does not affect the process of automatic semicolon
insertion (see 7.9).

Comments behave like white space and are discarded except that, if a MultiLineComment contains a
line terminator character, then the entire comment is considered to be a LineTerminator for purposes
of parsing by the syntactic grammar.

Identifier Names are tokens that are interpreted according to the grammar given in the “Identifiers” section of
chapter 5 of the Unicode standard, with some small modifications. An Identifier is an IdentifierName that is not a ReservedWord (see 7.6.1). The
Unicode identifier grammar is based on both normative and informative character categories specified by the Unicode Standard.
The characters in the specified categories in version 3.0 of the Unicode standard must be treated as in those categories by
all conforming ECMAScript implementations.

This standard specifies specific character additions: The dollar sign ($) and the underscore (_)
are permitted anywhere in an IdentifierName.

Unicode escape sequences are also permitted in an IdentifierName, where they contribute a single
character to the IdentifierName, as computed by the CV of the UnicodeEscapeSequence (see 7.8.4). The \ preceding the UnicodeEscapeSequence does not contribute a character to the IdentifierName. A UnicodeEscapeSequence cannot be used to put a character into an IdentifierName that
would otherwise be illegal. In other words, if a \UnicodeEscapeSequence sequence were
replaced by its UnicodeEscapeSequence's CV, the result must still be a valid IdentifierName that has the exact same sequence of characters as the original IdentifierName. All interpretations of identifiers within this specification are based upon their actual
characters regardless of whether or not an escape sequence was used to contribute any particular characters.

Two IdentifierName that are canonically equivalent according to the Unicode standard are not
equal unless they are represented by the exact same sequence of code units (in other words, conforming ECMAScript
implementations are only required to do bitwise comparison on IdentifierName values). The intent is that the incoming source
text has been converted to normalised form C before it reaches the compiler.

ECMAScript implementations may recognise identifier characters defined in later editions of the Unicode Standard. If
portability is a concern, programmers should only employ identifier characters defined in Unicode 3.0.

Syntax

The following words are used as keywords in proposed extensions and are therefore reserved to allow for the possibility
of future adoption of those extensions.

Syntax

FutureReservedWord::one of

class

enum

extends

super

const

export

import

The following tokens are also considered to be FutureReservedWords when they occur within strict mode code (see 10.1.1). The occurrence of any of these tokens within strict mode code in any context where the occurrence of a FutureReservedWord would produce an error must also produce an equivalent error:

The source character immediately following a NumericLiteral must not be an IdentifierStart or DecimalDigit.

NOTE For example:

3in

is an error and not the two input elements 3 and in.

Semantics

A numeric literal stands for a value of the Number type. This value is determined in two steps: first, a mathematical
value (MV) is derived from the literal; second, this mathematical value is rounded as described below.

The MV of NumericLiteral::DecimalLiteral is the MV of DecimalLiteral.

The MV of NumericLiteral::HexIntegerLiteral is the MV of HexIntegerLiteral.

The MV of DecimalLiteral::DecimalIntegerLiteral. is the MV of DecimalIntegerLiteral.

The MV of DecimalLiteral::DecimalIntegerLiteral.DecimalDigits is the MV
of DecimalIntegerLiteral plus (the MV of DecimalDigits times 10–n), where n
is the number of characters in DecimalDigits.

The MV of DecimalLiteral::DecimalIntegerLiteral.ExponentPart is the MV
of DecimalIntegerLiteral times 10e, where e is the MV of ExponentPart.

The MV of DecimalLiteral::DecimalIntegerLiteral.DecimalDigitsExponentPart is (the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits
times 10–n)) times 10e, where n is the number of characters in
DecimalDigits and e is the MV of ExponentPart.

The MV of DecimalLiteral::.DecimalDigits is the MV of DecimalDigits times
10–n, where n is the number of characters in DecimalDigits.

The MV of DecimalLiteral::.DecimalDigitsExponentPart is the MV of
DecimalDigits times 10e–n, where n is the number of characters in
DecimalDigits and e is the MV of ExponentPart.

The MV of DecimalLiteral::DecimalIntegerLiteral is the MV of DecimalIntegerLiteral.

The MV of DecimalLiteral::DecimalIntegerLiteralExponentPart is the MV of
DecimalIntegerLiteral times 10e, where e is the MV of ExponentPart.

The MV of DecimalIntegerLiteral::0 is 0.

The MV of DecimalIntegerLiteral::NonZeroDigit is the MV of NonZeroDigit.

The MV of DecimalIntegerLiteral::NonZeroDigitDecimalDigits is (the MV of NonZeroDigit times
10n) plus the MV of DecimalDigits, where n is the number of characters in
DecimalDigits.

The MV of DecimalDigits::DecimalDigit is the MV of DecimalDigit.

The MV of DecimalDigits::DecimalDigitsDecimalDigit is (the MV of DecimalDigits times 10)
plus the MV of DecimalDigit.

The MV of ExponentPart::ExponentIndicatorSignedInteger is the MV of SignedInteger.

The MV of SignedInteger::DecimalDigits is the MV of DecimalDigits.

The MV of SignedInteger::+DecimalDigits is the MV of DecimalDigits.

The MV of SignedInteger::-DecimalDigits is the negative of the MV of DecimalDigits.

The MV of DecimalDigit::0 or of HexDigit::0 is 0.

The MV of DecimalDigit::1 or of NonZeroDigit::1 or of HexDigit::1 is 1.

The MV of DecimalDigit::2 or of NonZeroDigit::2 or of HexDigit::2 is 2.

The MV of DecimalDigit::3 or of NonZeroDigit::3 or of HexDigit::3 is 3.

The MV of DecimalDigit::4 or of NonZeroDigit::4 or of HexDigit::4 is 4.

The MV of DecimalDigit::5 or of NonZeroDigit::5 or of HexDigit::5 is 5.

The MV of DecimalDigit::6 or of NonZeroDigit::6 or of HexDigit::6 is 6.

The MV of DecimalDigit::7 or of NonZeroDigit::7 or of HexDigit::7 is 7.

The MV of DecimalDigit::8 or of NonZeroDigit::8 or of HexDigit::8 is 8.

The MV of DecimalDigit::9 or of NonZeroDigit::9 or of HexDigit::9 is 9.

The MV of HexDigit::a or of HexDigit::A is 10.

The MV of HexDigit::b or of HexDigit::B is 11.

The MV of HexDigit::c or of HexDigit::C is 12.

The MV of HexDigit::d or of HexDigit::D is 13.

The MV of HexDigit::e or of HexDigit::E is 14.

The MV of HexDigit::f or of HexDigit::F is 15.

The MV of HexIntegerLiteral::0xHexDigit is the MV of HexDigit.

The MV of HexIntegerLiteral::0XHexDigit is the MV of HexDigit.

The MV of HexIntegerLiteral::HexIntegerLiteralHexDigit is (the MV of HexIntegerLiteral times
16) plus the MV of HexDigit.

Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type. If the MV
is 0, then the rounded value is +0; otherwise, the rounded value must be the Number value for the
MV (as specified in 8.5), unless the literal is a DecimalLiteral and the
literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a
literal produced by replacing each significant digit after the 20th with a 0 digit or the Number value for the
MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then
incrementing the literal at the 20th significant digit position. A digit is significant if it is not part of an ExponentPart and

it is not 0; or

there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.

A conforming implementation, when processing strict mode code (see 10.1.1), must not extend the
syntax of NumericLiteral to include OctalIntegerLiteral as described in B.1.1.

A string literal is zero or more characters enclosed in single or double quotes. Each character may be represented by an
escape sequence. All characters may appear literally in a string literal except for the closing quote character, backslash,
carriage return, line separator, paragraph separator, and line feed. Any character may appear in the form of an escape
sequence.

Syntax

StringLiteral::

"DoubleStringCharactersopt"

'SingleStringCharactersopt'

DoubleStringCharacters::

DoubleStringCharacterDoubleStringCharactersopt

SingleStringCharacters::

SingleStringCharacterSingleStringCharactersopt

DoubleStringCharacter::

SourceCharacterbut not one of"or\orLineTerminator

\EscapeSequence

LineContinuation

SingleStringCharacter::

SourceCharacterbut not one of'or\orLineTerminator

\EscapeSequence

LineContinuation

LineContinuation::

\LineTerminatorSequence

EscapeSequence::

CharacterEscapeSequence

0[lookahead ∉ DecimalDigit]

HexEscapeSequence

UnicodeEscapeSequence

CharacterEscapeSequence::

SingleEscapeCharacter

NonEscapeCharacter

SingleEscapeCharacter::one of

'"\bfnrtv

NonEscapeCharacter::

SourceCharacterbut not one ofEscapeCharacterorLineTerminator

EscapeCharacter::

SingleEscapeCharacter

DecimalDigit

x

u

HexEscapeSequence::

xHexDigitHexDigit

UnicodeEscapeSequence::

uHexDigitHexDigitHexDigitHexDigit

The definition of the nonterminal HexDigit is given in 7.8.3. SourceCharacter is defined in clause 6.

Semantics

A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms of
character values (CV) contributed by the various parts of the string literal. As part of this process, some characters
within the string literal are interpreted as having a mathematical value (MV), as described below or in 7.8.3.

The SV of StringLiteral::"" is the empty character sequence.

The SV of StringLiteral::'' is the empty character sequence.

The SV of StringLiteral::"DoubleStringCharacters" is the SV of
DoubleStringCharacters.

The SV of StringLiteral::'SingleStringCharacters' is the SV of
SingleStringCharacters.

The SV of DoubleStringCharacters::DoubleStringCharacter is a sequence of one character, the CV of
DoubleStringCharacter.

The SV of DoubleStringCharacters::DoubleStringCharacterDoubleStringCharacters is a sequence of the CV of
DoubleStringCharacter followed by all the characters in the SV of DoubleStringCharacters in order.

The SV of SingleStringCharacters::SingleStringCharacter is a sequence of one character, the CV of
SingleStringCharacter.

The SV of SingleStringCharacters::SingleStringCharacter SingleStringCharacters is a sequence of the CV of
SingleStringCharacter followed by all the characters in the SV of SingleStringCharacters in order.

The SV of LineContinuation::\LineTerminatorSequence is the empty character sequence.

The CV of DoubleStringCharacter::SourceCharacterbut not one of"or\orLineTerminator is the SourceCharacter character itself.

The CV of DoubleStringCharacter::\EscapeSequence is the CV of the EscapeSequence.

The CV of DoubleStringCharacter::LineContinuation is the empty character sequence.

The CV of SingleStringCharacter::SourceCharacterbut not one of'or\orLineTerminator is the SourceCharacter character itself.

The CV of SingleStringCharacter::\EscapeSequence is the CV of the EscapeSequence.

The CV of SingleStringCharacter::LineContinuation is the empty character sequence.

The CV of EscapeSequence::CharacterEscapeSequence is the CV of the CharacterEscapeSequence.

The CV of EscapeSequence::0[lookahead ∉ DecimalDigit] is a
<NUL> character (Unicode value 0000).

The CV of EscapeSequence::HexEscapeSequence is the CV of the HexEscapeSequence.

The CV of EscapeSequence::UnicodeEscapeSequence is the CV of the UnicodeEscapeSequence.

The CV of CharacterEscapeSequence::SingleEscapeCharacter is the character whose code unit value is determined by the
SingleEscapeCharacter according to Table 4:

Table 4 — String Single Character Escape Sequences

Escape Sequence

Code Unit Value

Name

Symbol

\b

\u0008

backspace

<BS>

\t

\u0009

horizontal tab

<HT>

\n

\u000A

line feed (new line)

<LF>

\v

\u000B

vertical tab

<VT>

\f

\u000C

form feed

<FF>

\r

\u000D

carriage return

<CR>

\"

\u0022

double quote

"

\'

\u0027

single quote

'

\\

\u005C

backslash

\

The CV of CharacterEscapeSequence::NonEscapeCharacter is the CV of the NonEscapeCharacter.

The CV of NonEscapeCharacter::SourceCharacterbut not one ofEscapeCharacterorLineTerminator is the SourceCharacter character
itself.

The CV of HexEscapeSequence::xHexDigitHexDigit is the character whose code
unit value is (16 times the MV of the first HexDigit) plus the MV of the second HexDigit.

The CV of UnicodeEscapeSequence::uHexDigitHexDigitHexDigitHexDigit is the character whose code unit value is (4096 times the MV of the first
HexDigit) plus (256 times the MV of the second HexDigit) plus (16 times the MV of the third
HexDigit) plus the MV of the fourth HexDigit.

A conforming implementation, when processing strict mode code (see 10.1.1), may not extend the
syntax of EscapeSequence to include OctalEscapeSequence as described in B.1.2.

NOTE A line terminator character cannot appear in a string literal, except as part of a LineContinuation to produce the empty character sequence. The correct way to cause a line terminator
character to be part of the String value of a string literal is to use an escape sequence such as \n or
\u000A.

A regular expression literal is an input element that is converted to a RegExp object (see
15.10) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression
objects that never compare as === to each other even if the two literals' contents are identical. A RegExp
object may also be created at runtime by new RegExp (see 15.10.4) or calling the
RegExp constructor as a function (15.10.3).

The productions below describe the syntax for a regular expression literal and are used by the input element scanner to
find the end of the regular expression literal. The Strings of characters comprising the RegularExpressionBody and the RegularExpressionFlags are passed uninterpreted to
the regular expression constructor, which interprets them according to its own, more stringent grammar. An implementation
may extend the regular expression constructor's grammar, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions or the productions
used by these productions.

Semantics

A regular expression literal evaluates to a value of the Object type that is an instance of the standard built-in
constructor RegExp. This value is determined in two steps: first, the characters comprising the regular expression's RegularExpressionBody and RegularExpressionFlags production expansions are
collected uninterpreted into two Strings Pattern and Flags, respectively. Then each time the literal is evaluated, a new
object is created as if by the expression new RegExp(Pattern,
Flags) where RegExp is the standard built-in constructor with that name. The newly constructed object becomes
the value of the RegularExpressionLiteral. If the call to new RegExp would generate an
error as specified in 15.10.4.1, the error must be treated as an early error (Clause 16).

Certain ECMAScript statements (empty statement, variable statement, expression statement,
do-while statement, continue statement, break statement,
return statement, and throw statement) must be terminated with semicolons. Such semicolons may
always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in
certain situations. These situations are described by saying that semicolons are automatically inserted into the source code
token stream in those situations.

When, as the program is parsed from left to right, a token (called the offending token) is encountered that is
not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if
one or more of the following conditions is true:

The offending token is separated from the previous token by at least one LineTerminator.

The offending token is }.

When, as the program is parsed from left to right, the end of the input stream of tokens is encountered and the parser
is unable to parse the input token stream as a single complete ECMAScript Program, then a
semicolon is automatically inserted at the end of the input stream.

When, as the program is parsed from left to right, a token is encountered that is allowed by some production of the
grammar, but the production is a restricted production and the token would be the first token for a terminal or
nonterminal immediately following the annotation “[no
LineTerminator here]” within the restricted production
(and therefore such a token is called a restricted token), and the restricted token is separated from the previous
token by at least one LineTerminator, then a semicolon is automatically inserted before the
restricted token.

However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically
if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in
the header of a for statement (see 12.6.3).

NOTE The following are the only restricted productions in the grammar:

PostfixExpression:

LeftHandSideExpression[no LineTerminator here]++

LeftHandSideExpression[no LineTerminator here]--

ContinueStatement:

continue[no LineTerminator here]Identifier;

BreakStatement:

break[no LineTerminator here]Identifier;

ReturnStatement:

return[no LineTerminator here]Expression;

ThrowStatement:

throw[no LineTerminator here]Expression;

The practical effect of these restricted productions is as follows:

When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and
at least one LineTerminator occurred between the preceding token and the ++ or
-- token, then a semicolon is automatically inserted before the ++ or -- token.

When a continue, break, return, or throw token is encountered and a
LineTerminator is encountered before the next token, a semicolon is automatically inserted after the
continue, break, return, or throw token.

The resulting practical advice to ECMAScript programmers is:

A postfix ++ or -- operator should appear on the same line as its operand.

An Expression in a return or throw statement should start on the same
line as the return or throw token.

An Identifier in a break or continue statement should be on the same
line as the break or continue token.

is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the
source

{ 12 } 3

is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:

{ 1;2 ;} 3;

which is a valid ECMAScript sentence.

The source

for (a; b)

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed
for the header of a for statement. Automatic semicolon insertion never inserts one of the two semicolons in the
header of a for statement.

The source

returna + b

is transformed by automatic semicolon insertion into the following:

return;a + b;

NOTE The expression a + b is not treated as a value to be returned by the
return statement, because a LineTerminator separates it from the token
return.

The source

a = b++c

is transformed by automatic semicolon insertion into the following:

a = b;++c;

NOTE The token ++ is not treated as a postfix operator applying to the variable
b, because a LineTerminator occurs between b and ++.

The source

if (a > b)else c = d

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else
token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would
then be parsed as an empty statement.

The source

a = b + c(d + e).print()

is not transformed by automatic semicolon insertion, because the parenthesised expression that begins the second
line can be interpreted as an argument list for a function call:

a = b + c(d + e).print()

In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer
to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon
insertion.

Algorithms within this specification manipulate values each of which has an associated type. The possible value types are
exactly those defined in this clause. Types are further subclassified into ECMAScript language types and specification
types.

An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the
ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean, String, Number, and Object.

A specification type corresponds to meta-values that are used within algorithms to describe the semantics of ECMAScript
language constructs and ECMAScript language types. The specification types are Reference, List, Completion, Property Descriptor, Property Identifier, Lexical Environment, and Environment
Record. Specification type values are specification artefacts that do not necessarily correspond to any specific entity
within an ECMAScript implementation. Specification type values may be used to describe intermediate results of ECMAScript
expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript language variables.

Within this specification, the notation “Type(x)” is
used as shorthand for “the type of x” where “type” refers to the ECMAScript language and specification types defined in
this clause.

The String type is the set of all finite ordered sequences of zero or more 16-bit unsigned integer values
(“elements”). The String type is generally used to represent textual data in a running ECMAScript program, in
which case each element in the String is treated as a code unit value (see Clause 6). Each element is
regarded as occupying a position within the sequence. These positions are indexed with nonnegative integers. The first element
(if any) is at position 0, the next element (if any) at position 1, and so on. The length of a String is the number of
elements (i.e., 16-bit values) within it. The empty String has length zero and therefore contains no elements.

When a String contains actual textual data, each element is considered to be a single UTF-16 code unit. Whether or not this
is the actual storage format of a String, the characters within a String are numbered by their initial code unit element
position as though they were represented using UTF-16. All operations on Strings (except as otherwise stated) treat them as
sequences of undifferentiated 16-bit unsigned integers; they do not ensure the resulting String is in normalised form, nor do
they ensure language-sensitive results.

NOTE The rationale behind this design was to keep the implementation of Strings as simple and
high-performing as possible. The intent is that textual data coming into the execution environment from outside (e.g., user
input, text read from a file or received over the network, etc.) be converted to Unicode Normalised Form C before the
running program sees it. Usually this would occur at the same time incoming text is converted from its original character
encoding to Unicode (and would impose no additional overhead). Since it is recommended that ECMAScript source code be in
Normalised Form C, string literals are guaranteed to be normalised (if source text is guaranteed to be normalised), as long
as they do not contain any Unicode escape sequences.

The Number type has exactly 18437736874454810627 (that is, 264−253+3) values, representing the double-precision
64-bit format IEEE 754 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9007199254740990 (that is, 253−2) distinct “Not-a-Number” values of the IEEE Standard are represented in
ECMAScript as a single special NaN value. (Note that the NaN value is produced by the program expression
NaN.) In some implementations, external code might be able to detect a difference between various Not-a-Number
values, but such behaviour is implementation-dependent; to ECMAScript code, all NaN values are indistinguishable from each
other.

There are two other special values, called positive Infinity and negative Infinity. For brevity, these values
are also referred to for expository purposes by the symbols +∞ and −∞, respectively. (Note that these two infinite Number values are produced by the program
expressions +Infinity (or simply Infinity) and -Infinity.)

The other 18437736874454810624 (that is, 264−253) values are called the finite numbers. Half of these are positive
numbers and half are negative numbers; for every finite positive Number value there is a corresponding negative value having
the same magnitude.

Note that there is both a positive zero and a negative zero. For brevity, these values are also referred to
for expository purposes by the symbols +0 and −0, respectively.
(Note that these two different zero Number values are produced by the program expressions +0 (or simply
0) and -0.)

The 18437736874454810622 (that is, 264−253−2) finite nonzero values are of two kinds:

18428729675200069632 (that is, 264−254) of them are normalised, having the form

s × m × 2e

where s is +1 or −1, m is a positive integer less than 253 but not less than 252, and
e is an integer ranging from −1074 to 971, inclusive.

The remaining 9007199254740990 (that is, 253−2) values are denormalised, having the form

s × m × 2e

where s is +1 or −1, m is a positive integer less than 252, and e is −1074.

Note that all the positive and negative integers whose magnitude is no greater than 253 are representable in the Number type (indeed, the integer 0 has two representations, +0 and -0).

A finite number has an odd significand if it is nonzero and the integer m used to express it (in one of
the two forms shown above) is odd. Otherwise, it has an even significand.

In this specification, the phrase “the Number value for
x” where x represents an exact nonzero real mathematical quantity (which might even be an
irrational number such as π) means a Number value chosen in the following
manner. Consider the set of all finite values of the Number type, with −0 removed and with
two additional values added to it that are not representable in the Number type, namely 21024 (which is +1 × 253× 2971) and −21024 (which is −1 ×
253× 2971). Choose the member of this
set that is closest in value to x. If two values of the set are equally close, then the one with an even
significand is chosen; for this purpose, the two extra values 21024 and −21024 are considered to
have even significands. Finally, if 21024 was chosen, replace it
with +∞; if −21024 was
chosen, replace it with −∞; if +0 was chosen, replace it
with −0 if and only if x is less than zero; any other chosen value is used
unchanged. The result is the Number value for x. (This procedure corresponds exactly to the behaviour of the IEEE
754 “round to nearest” mode.)

Some ECMAScript operators deal only with integers in the range −231 through 231−1,
inclusive, or in the range 0 through 232−1, inclusive. These operators accept any value of the Number type but first convert each
such value to one of 232 integer values. See the descriptions of
the ToInt32 and ToUint32 operators in 9.5 and 9.6, respectively.

An Object is a collection of properties. Each property is either a named data property, a named accessor property, or an
internal property:

A named data property associates a name with an ECMAScript language value and a set of Boolean attributes.

A named accessor property associates a name with one or two accessor functions, and a set of Boolean attributes.
The accessor functions are used to store or retrieve an ECMAScript language value that is associated with the
property.

An internal property has no name and is not directly accessible via ECMAScript language operators. Internal
properties exist purely for specification purposes.

There are two kinds of access for named (non-internal) properties: get and put, corresponding to retrieval
and assignment, respectively.

Attributes are used in this specification to define and explain the state of named properties. A named data property
associates a name with the attributes listed in Table 5

Table 5 — Attributes of a Named Data Property

Attribute Name

Value Domain

Description

[[Value]]

Any ECMAScript language type

The value retrieved by reading the property.

[[Writable]]

Boolean

If false, attempts by ECMAScript code to change the property’s [[Value]] attribute using [[Put]] will not succeed.

[[Enumerable]]

Boolean

If true, the property will be enumerated by a for-in enumeration (see 12.6.4). Otherwise, the property is said to be non-enumerable.

[[Configurable]]

Boolean

If false, attempts to delete the property, change the property to be an accessor property, or change its attributes (other than [[Value]]) will fail.

A named accessor property associates a name with the attributes listed in Table 6.

Table 6 — Attributes of a Named Accessor Property

Attribute Name

Value Domain

Description

[[Get]]

Object or Undefined

If the value is an Object it must be a function Object. The function’s [[Call]] internal method (8.6.2) is called with an empty arguments list to return the property value each time a get access of the property is performed.

[[Set]]

Object or Undefined

If the value is an Object it must be a function Object. The function’s [[Call]] internal method (8.6.2) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property's [[Set]] internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property's [[Get]] internal method.

[[Enumerable]]

Boolean

If true, the property is to be enumerated by a for-in enumeration (see 12.6.4). Otherwise, the property is said to be non-enumerable.

[[Configurable]]

Boolean

If false, attempts to delete the property, change the property to be a data property, or change its attributes will fail.

If the value of an attribute is not explicitly specified by this specification for a named property, the default value
defined in Table 7 is used.

This specification uses various internal properties to define the semantics of object values. These internal properties
are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. An
implementation of ECMAScript must behave as if it produced and operated upon internal properties in the manner described
here. The names of internal properties are enclosed in double square brackets [[ ]]. When an algorithm uses an internal
property of an object and the object does not implement the indicated internal property, a TypeError exception is
thrown.

The Table 8 summarises the internal properties used by this specification that are applicable to all ECMAScript objects.
The Table 9 summarises the internal properties used by this specification that are only applicable to some ECMAScript
objects. The descriptions in these tables indicate their behaviour for native ECMAScript objects, unless stated otherwise in
this document for particular kinds of native ECMAScript objects. Host objects may support these internal properties with any
implementation-dependent behaviour as long as it is consistent with the specific host object restrictions stated in this
document.

The “Value Type Domain” columns of the following tables define the types of values associated with internal
properties. The type names refer to the types defined in Clause 8 augmented by the following additional
names. “any” means the value may be any ECMAScript language type. “primitive” means
Undefined, Null, Boolean, String, or Number. “SpecOp” means the internal property is an internal method,
an implementation provided procedure defined by an abstract operation specification. “SpecOp” is followed by a
list of descriptive parameter names. If a parameter name is the same as a type name then the name describes the type of the
parameter. If a “SpecOp” returns a value, its parameter list is followed by the symbol “→” and
the type of the returned value.

Table 8 — Internal Properties Common to All Objects

Internal Property

Value Type Domain

Description

[[Prototype]]

Object or Null

The prototype of this object.

[[Class]]

String

A String value indicating a specification defined classification of objects.

Returns the fully populated Property Descriptor of the named property of this object, or undefined if absent.

[[Put]]

SpecOp (propertyName, any, Boolean)

Sets the specified named property to the value of the second parameter. The flag controls failure handling.

[[CanPut]]

SpecOp (propertyName) → Boolean

Returns a Boolean value indicating whether a [[Put]] operation with PropertyName can be performed.

[[HasProperty]]

SpecOp (propertyName) →Boolean

Returns a Boolean value indicating whether the object already has a property with the given name.

[[Delete]]

SpecOp (propertyName, Boolean) → Boolean

Removes the specified named own property from the object. The flag controls failure handling.

[[DefaultValue]]

SpecOp (Hint) →primitive

Hint is a String. Returns a default value for the object.

[[DefineOwnProperty]]

SpecOp (propertyName, PropertyDescriptor, Boolean) → Boolean

Creates or alters the named own property to have the state described by a Property Descriptor. The flag controls failure handling.

Every object (including host objects) must implement all of the internal properties listed in Table 8. However, the
[[DefaultValue]] internal method may, for some objects, simply throw a TypeError exception.

All objects have an internal property called [[Prototype]]. The value of this property is either null or an object
and is used for implementing inheritance. Whether or not a native object can have a host object as its [[Prototype]] depends
on the implementation. Every [[Prototype]] chain must have finite length (that is, starting from any object, recursively
accessing the [[Prototype]] internal property must eventually lead to a null value). Named data properties of the
[[Prototype]] object are inherited (are visible as properties of the child object) for the purposes of get access, but not
for put access. Named accessor properties are inherited for both get access and put access.

Every ECMAScript object has a Boolean-valued [[Extensible]] internal property that controls whether or not named
properties may be added to the object. If the value of the [[Extensible]] internal property is false then additional
named properties may not be added to the object. In addition, if [[Extensible]] is false the value of the [[Class]]
and [[Prototype]] internal properties of the object may not be modified. Once the value of an [[Extensible]] internal
property has been set to false it may not be subsequently changed to true.

NOTE This specification defines no ECMAScript language operators or built-in functions that
permit a program to modify an object’s [[Class]] or [[Prototype]] internal properties or to change the value of
[[Extensible]] from false to true. Implementation specific extensions that modify [[Class]], [[Prototype]]
or [[Extensible]] must not violate the invariants defined in the preceding paragraph.

The value of the [[Class]] internal property is defined by this specification for every kind of built-in object. The
value of the [[Class]] internal property of a host object may be any String value except one of "Arguments",
"Array", "Boolean", "Date", "Error", "Function",
"JSON", "Math", "Number", "Object", "RegExp", and
"String". The value of a [[Class]] internal property is used internally to distinguish different kinds of
objects. Note that this specification does not provide any means for a program to access that value except through
Object.prototype.toString (see 15.2.4.2).

Unless otherwise specified, the common internal methods of native ECMAScript objects behave as described in 8.12. Array objects have a slightly different implementation of the [[DefineOwnProperty]] internal
method (see 15.4.5.1) and String objects have a slightly different implementation of the
[[GetOwnProperty]] internal method (see 15.5.5.2). Arguments objects (10.6) have different implementations of [[Get]], [[GetOwnProperty]], [[DefineOwnProperty]], and
[[Delete]]. Function objects (15.3) have a different implementation of [[Get]].

Host objects may implement these internal methods in any manner unless specified otherwise; for example, one possibility
is that [[Get]] and [[Put]] for a particular host object indeed fetch and store property values but [[HasProperty]] always
generates false. However, if any specified manipulation of a host object's internal properties is not supported by an
implementation, that manipulation must throw a TypeError exception when attempted.

The [[GetOwnProperty]] internal method of a host object must conform to the following invariants for each property of the
host object:

If a property is described as a data property and it may return different values over time, then either or both of
the [[Writable]] and [[Configurable]] attributes must be true even if no mechanism to change the value is exposed
via the other internal methods.

If a property is described as a data property and its [[Writable]] and [[Configurable]] are both false, then
the SameValue (according to 9.12) must be returned for the [[Value]] attribute of the property
on all calls to [[GetOwnProperty]].

If the attributes other than [[Writable]] may change over time or if the property might disappear, then the
[[Configurable]] attribute must be true.

If the [[Writable]] attribute may change from false to true, then the [[Configurable]] attribute must
be true.

If the value of the host object’s [[Extensible]] internal property has been observed by ECMAScript code to be
false, then if a call to [[GetOwnProperty]] describes a property as non-existent all subsequent calls must also
describe that property as non-existent.

The [[DefineOwnProperty]] internal method of a host object must not permit the addition of a new property to a host
object if the [[Extensible]] internal property of that host object has been observed by ECMAScript code to be
false.

If the [[Extensible]] internal property of that host object has been observed by ECMAScript code to be false then
it must not subsequently become true.

Table 9 — Internal Properties Only Defined for Some Objects

Internal Property

Value Type Domain

Description

[[PrimitiveValue]]

primitive

Internal state information associated with this object. Of the standard built-in ECMAScript objects, only Boolean, Date, Number, and String objects implement [[PrimitiveValue]].

Executes code associated with the object. Invoked via a function call expression. The arguments to the SpecOp are this object and a list containing the arguments passed to the function call expression. Objects that implement this internal method are callable. Only callable objects that are host objects may return Reference values.

[[HasInstance]]

SpecOp(any) → Boolean

Returns a Boolean value indicating whether the argument is likely an Object that was constructed by this object. Of the standard built-in ECMAScript objects, only Function objects implement [[HasInstance]].

A possibly empty List containing the identifier Strings of a Function’s FormalParameterList. Of the standard built-in ECMAScript objects, only Function objects implement [[FormalParameterList]].

[[Code]]

ECMAScript code

The ECMAScript code of a function. Of the standard built-in ECMAScript objects, only Function objects implement [[Code]].

[[TargetFunction]]

Object

The target function of a function object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[TargetFunction]] internal property.

[[BoundThis]]

any

The pre-bound this value of a function Object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[BoundThis]] internal property.

The pre-bound argument values of a function Object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[BoundArguments]] internal property.

Provides a mapping between the properties of an arguments object (see 10.6) and the formal parameters of the associated function. Only ECMAScript objects that are arguments objects have a [[ParameterMap]] internal property.

The Reference type is used to explain the behaviour of such operators as delete, typeof, and the
assignment operators. For example, the left-hand operand of an assignment is expected to produce a reference. The behaviour of
assignment could, instead, be explained entirely in terms of a case analysis on the syntactic form of the left-hand operand of
an assignment operator, but for one difficulty: function calls are permitted to return references. This possibility is
admitted purely for the sake of host objects. No built-in ECMAScript function defined by this specification returns a
reference and there is no provision for a user-defined function to return a reference. (Another reason not to use a syntactic
case analysis is that it would be lengthy and awkward, affecting many parts of the specification.)

A Reference is a resolved name binding. A Reference consists of three components, the base value, the
referenced name and the Boolean valued strict reference flag. The base value is either undefined,
an Object, a Boolean, a String, a Number, or an environment record (10.2.1). A base value of
undefined indicates that the reference could not be resolved to a binding. The referenced name is a String.

The following abstract operations are used in this specification to access the components of references:

GetBase(V). Returns the base value component of the reference V.

GetReferencedName(V). Returns the referenced name component of the reference V.

IsStrictReference(V). Returns the strict reference component of the reference V.

HasPrimitiveBase(V). Returns true if the base value is a Boolean, String, or Number.

IsPropertyReference(V). Returns true if either the base value is an object or HasPrimitiveBase(V) is
true; otherwise returns false.

IsUnresolvableReference(V). Returns true if the base value is undefined and false otherwise.

The following abstract operations are used in this specification to operate on references:

The following [[Get]] internal method is used by GetValue when V is a property reference with a primitive base
value. It is called using base as its this value and with property P as its argument. The
following steps are taken:

Return the result calling the [[Call]] internal method of getter providing base as the this value
and providing no arguments.

NOTE The object that may be created in step 1 is not accessible outside of the above method. An
implementation might choose to avoid the actual creation of the object. The only situation where such an actual property
access that uses this internal method can have visible effect is when it invokes an accessor function.

The following [[Put]] internal method is used by PutValue when V is a property reference with a primitive base
value. It is called using base as its this value and with property P, value W, and
Boolean flag Throw as arguments. The following steps are taken:

Let desc be the result of calling the [[GetProperty]] internal method of O with argument P. This
may be either an own or inherited accessor property descriptor or an inherited data property descriptor.

Call the [[Call]] internal method of setter providing base as the this value and an argument
list containing only W.

Else, this is a request to create an own property on the transient object O

If Throw is true, then throw a TypeError exception.

Return.

NOTE The object that may be created in step 1 is not accessible outside of the above method.
An implementation might choose to avoid the actual creation of that transient object. The only situations where such an
actual property assignment that uses this internal method can have visible effect are when it either invokes an accessor
function or is in violation of a Throw predicated error check. When Throw
is true any property assignment that would create a new property on the transient object throws an error.

The List type is used to explain the evaluation of argument lists (see 11.2.4) in
new expressions, in function calls, and in other algorithms where a simple list of values is needed. Values of
the List type are simply ordered sequences of values. These sequences may be of any length.

The Completion type is used to explain the behaviour of statements (break, continue,
return and throw) that perform nonlocal transfers of control. Values of the Completion type are
triples of the form (type, value, target), where type is one of normal, break,
continue, return, or throw, value is any ECMAScript language value or empty, and
target is any ECMAScript identifier or empty. If cv is a completion value then cv.type, cv.value,
and cv.target may be used to directly refer to its constituent
values.

The term “abrupt completion” refers to any completion with a type other than normal.

The Property Descriptor type is used to explain the manipulation and reification of named property attributes. Values of
the Property Descriptor type are records composed of named fields where each field’s name is an attribute name and its
value is a corresponding attribute value as specified in 8.6.1. In addition, any field may be present
or absent.

Property Descriptor values may be further classified as data property descriptors and accessor property descriptors based
upon the existence or use of certain fields. A data property descriptor is one that includes any fields named either [[Value]]
or [[Writable]]. An accessor property descriptor is one that includes any fields named either [[Get]] or [[Set]]. Any property
descriptor may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be both a data
property descriptor and an accessor property descriptor; however, it may be neither. A generic property descriptor is a
Property Descriptor value that is neither a data property descriptor nor an accessor property descriptor. A fully populated
property descriptor is one that is either an accessor property descriptor or a data property descriptor and that has all of
the fields that correspond to the property attributes defined in either 8.6.1 Table 5 or Table 6.

For notational convenience within this specification, an object literal-like syntax can be used to define a property
descriptor value. For example, Property Descriptor {[[Value]]: 42, [[Writable]]: false, [[Configurable]]: true}
defines a data property descriptor. Field name order is not significant. Any fields that are not explicitly listed are
considered to be absent.

In specification text and algorithms, dot notation may be used to refer to a specific field of a Property Descriptor. For
example, if D is a property descriptor then D.[[Value]] is shorthand for “the field of D named [[Value]]”.

The Property Identifier type is used to associate a property name with a Property Descriptor. Values of the Property
Identifier type are pairs of the form (name, descriptor), where name is a String and descriptor is a Property Descriptor
value.

The following abstract operations are used in this specification to operate upon Property Descriptor values:

Let desc be the result of calling the [[GetProperty]] internal method of O with argument P. This
may be either an own or inherited accessor property descriptor or an inherited data property descriptor.

Let str be the result of calling the [[Call]] internal method of toString, with O as the this
value and an empty argument list.

If str is a primitive value, return str.

Throw a TypeError exception.

When the [[DefaultValue]] internal method of O is called with no hint, then it behaves as if the hint were
Number, unless O is a Date object (see 15.9.6), in which case it behaves as if the hint
were String.

The above specification of [[DefaultValue]] for native objects can return only primitive values. If a host object
implements its own [[DefaultValue]] internal method, it must ensure that its [[DefaultValue]] internal method can return
only primitive values.

In the following algorithm, the term “Reject” means “If Throw is true,
then throw a TypeError exception, otherwise return false”. The algorithm contains steps that test
various fields of the Property DescriptorDesc for specific values. The
fields that are tested in this manner need not actually exist in Desc. If a field is absent then
its value is considered to be false.

When the [[DefineOwnProperty]] internal method of O is called with property name P, property
descriptor Desc, and Boolean flag Throw, the following steps are taken:

Let current be the result of calling the [[GetOwnProperty]] internal method of O with property name
P.

Let extensible be the value of the [[Extensible]] internal property of O.

Create an own data property named P of object O whose [[Value]], [[Writable]], [[Enumerable]]
and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of
Desc is absent, the attribute of the newly created property is set to its default value.

Create an own accessor property named P of object O whose [[Get]], [[Set]], [[Enumerable]] and
[[Configurable]] attribute values are described by Desc. If the value of an attribute field of
Desc is absent, the attribute of the newly created property is set to its default value.

Return true.

Return true, if every field in Desc is absent.

Return true, if every field in Desc also occurs in current and the value of every field in
Desc is the same value as the corresponding field in current when compared using the
SameValue algorithm (9.12).

If the [[Configurable]] field of current is false then

Reject, if the [[Configurable]] field of Desc is true.

Reject, if the [[Enumerable]] field of Desc is present and the [[Enumerable]] fields of current and
Desc are the Boolean negation of each other.

Convert the property named P of object O from a data property to an accessor property. Preserve
the existing values of the converted property’s [[Configurable]] and [[Enumerable]] attributes and set
the rest of the property’s attributes to their default values.

Else,

Convert the property named P of object O from an accessor property to a data property. Preserve
the existing values of the converted property’s [[Configurable]] and [[Enumerable]] attributes and set
the rest of the property’s attributes to their default values.

Reject, if the [[Set]] field of Desc is present and SameValue(Desc.[[Set]], current.[[Set]]) is false.

Reject, if the [[Get]] field of Desc is present and SameValue(Desc.[[Get]], current.[[Get]]) is false.

For each attribute field of Desc that is present, set the correspondingly named attribute of the property named
P of object O to the value of the field.

Return true.

However, if O is an Array object, it has a more elaborate [[DefineOwnProperty]] internal method defined in 15.4.5.1.

NOTE Step 10.b allows any field of Desc to be different from the corresponding field of current
if current’s [[Configurable]] field is true. This even permits changing the [[Value]] of a property whose
[[Writable]] attribute is false. This is allowed because a true [[Configurable]] attribute would permit an
equivalent sequence of calls where [[Writable]] is first set to true, a new [[Value]] is set, and then [[Writable]]
is set to false.

The ECMAScript runtime system performs automatic type conversion as needed. To clarify the semantics of certain constructs it
is useful to define a set of conversion abstract operations. These abstract operations are not a part of the language; they are
defined here to aid the specification of the semantics of the language. The conversion abstract operations are polymorphic; that
is, they can accept a value of any ECMAScript language type, but not of specification types.

The abstract operation ToPrimitive takes an input argument and an optional argument PreferredType. The abstract operation ToPrimitive converts its input argument to a non-Object
type. If an object is capable of converting to more than one primitive type, it may use the optional hint PreferredType to favour that type. Conversion occurs according to Table 10:

Table 10 — ToPrimitive Conversions

Input Type

Result

Undefined

The result equals the input argument (no conversion).

Null

The result equals the input argument (no conversion).

Boolean

The result equals the input argument (no conversion).

Number

The result equals the input argument (no conversion).

String

The result equals the input argument (no conversion).

Object

Return a default value for the Object. The default value of an object is retrieved by calling the [[DefaultValue]] internal method of the object, passing the optional hint PreferredType. The behaviour of the [[DefaultValue]] internal method is defined by this specification for all native ECMAScript objects in 8.12.8.

ToNumber applied to Strings applies the following grammar to the input String. If the grammar
cannot interpret the String as an expansion of StringNumericLiteral, then the result of ToNumber is NaN.

Syntax

StringNumericLiteral:::

StrWhiteSpaceopt

StrWhiteSpaceoptStrNumericLiteralStrWhiteSpaceopt

StrWhiteSpace:::

StrWhiteSpaceCharStrWhiteSpaceopt

StrWhiteSpaceChar:::

WhiteSpace

LineTerminator

StrNumericLiteral:::

StrDecimalLiteral

HexIntegerLiteral

StrDecimalLiteral:::

StrUnsignedDecimalLiteral

+StrUnsignedDecimalLiteral

-StrUnsignedDecimalLiteral

StrUnsignedDecimalLiteral:::

Infinity

DecimalDigits.DecimalDigitsoptExponentPartopt

.DecimalDigitsExponentPartopt

DecimalDigitsExponentPartopt

DecimalDigits:::

DecimalDigit

DecimalDigitsDecimalDigit

DecimalDigit:::one of

0123456789

ExponentPart:::

ExponentIndicatorSignedInteger

ExponentIndicator:::one of

eE

SignedInteger:::

DecimalDigits

+DecimalDigits

-DecimalDigits

HexIntegerLiteral:::

0xHexDigit

0XHexDigit

HexIntegerLiteralHexDigit

HexDigit:::one of

0123456789abcdefABCDEF

Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral (see 7.8.3):

A StringNumericLiteral may be preceded and/or followed by white space and/or line
terminators.

A StringNumericLiteral that is decimal may have any number of leading 0
digits.

A StringNumericLiteral that is decimal may be preceded by + or - to
indicate its sign.

A StringNumericLiteral that is empty or contains only white space is converted to
+0.

The conversion of a String to a Number value is similar overall to the determination of the Number value for a numeric
literal (see 7.8.3), but some of the details are different, so the process for converting a String
numeric literal to a value of Number type is given here in full. This value is determined in two steps: first, a
mathematical value (MV) is derived from the String numeric literal; second, this mathematical value is rounded as described
below.

The MV of StringNumericLiteral:::[empty] is 0.

The MV of StringNumericLiteral:::StrWhiteSpace is 0.

The MV of StringNumericLiteral:::StrWhiteSpaceoptStrNumericLiteralStrWhiteSpaceopt is the MV of StrNumericLiteral, no matter
whether white space is present or not.

The MV of StrNumericLiteral:::StrDecimalLiteral is the MV of StrDecimalLiteral.

The MV of StrNumericLiteral:::HexIntegerLiteral is the MV of HexIntegerLiteral.

The MV of StrDecimalLiteral:::StrUnsignedDecimalLiteral is the MV of StrUnsignedDecimalLiteral.

The MV of StrDecimalLiteral:::+StrUnsignedDecimalLiteral is the MV of StrUnsignedDecimalLiteral.

The MV of StrDecimalLiteral:::-StrUnsignedDecimalLiteral is the negative of the MV of StrUnsignedDecimalLiteral. (Note that if the MV of StrUnsignedDecimalLiteral
is 0, the negative of this MV is also 0. The rounding rule described below handles the conversion of this signless
mathematical zero to a floating-point +0 or −0 as appropriate.)

The MV of StrUnsignedDecimalLiteral:::Infinity is 1010000 (a value so
large that it will round to +∞).

The MV of StrUnsignedDecimalLiteral:::DecimalDigits. is the MV of DecimalDigits.

The MV of StrUnsignedDecimalLiteral:::DecimalDigits.DecimalDigits is the MV of the
first DecimalDigits plus (the MV of the second DecimalDigits times 10−n), where n is the number of
characters in the second DecimalDigits.

The MV of StrUnsignedDecimalLiteral:::DecimalDigits.ExponentPart is the MV of
DecimalDigits times 10e, where e is the MV of ExponentPart.

The MV of StrUnsignedDecimalLiteral:::DecimalDigits.DecimalDigitsExponentPart is (the MV of the first DecimalDigits plus (the MV of the second
DecimalDigits times 10−n)) times 10e, where n is the number of
characters in the second DecimalDigits and e is the MV of ExponentPart.

The MV of StrUnsignedDecimalLiteral:::.DecimalDigits is the MV of DecimalDigits times
10−n, where n is the number of characters in DecimalDigits.

The MV of StrUnsignedDecimalLiteral:::.DecimalDigitsExponentPart is the MV of
DecimalDigits times 10e−n, where n is the number of characters in
DecimalDigits and e is the MV of ExponentPart.

The MV of StrUnsignedDecimalLiteral:::DecimalDigits is the MV of DecimalDigits.

The MV of StrUnsignedDecimalLiteral:::DecimalDigitsExponentPart is the MV of DecimalDigits times
10e, where e is the MV of ExponentPart.

The MV of DecimalDigits:::DecimalDigit is the MV of DecimalDigit.

The MV of DecimalDigits:::DecimalDigitsDecimalDigit is (the MV of DecimalDigits times 10)
plus the MV of DecimalDigit.

The MV of ExponentPart:::ExponentIndicatorSignedInteger is the MV of SignedInteger.

The MV of SignedInteger:::DecimalDigits is the MV of DecimalDigits.

The MV of SignedInteger:::+DecimalDigits is the MV of DecimalDigits.

The MV of SignedInteger:::-DecimalDigits is the negative of the MV of DecimalDigits.

The MV of DecimalDigit:::0 or of HexDigit:::0 is 0.

The MV of DecimalDigit:::1 or of HexDigit:::1 is 1.

The MV of DecimalDigit:::2 or of HexDigit:::2 is 2.

The MV of DecimalDigit:::3 or of HexDigit:::3 is 3.

The MV of DecimalDigit:::4 or of HexDigit:::4 is 4.

The MV of DecimalDigit:::5 or of HexDigit:::5 is 5.

The MV of DecimalDigit:::6 or of HexDigit:::6 is 6.

The MV of DecimalDigit:::7 or of HexDigit:::7 is 7.

The MV of DecimalDigit:::8 or of HexDigit:::8 is 8.

The MV of DecimalDigit:::9 or of HexDigit:::9 is 9.

The MV of HexDigit:::a or of HexDigit:::A is 10.

The MV of HexDigit:::b or of HexDigit:::B is 11.

The MV of HexDigit:::c or of HexDigit:::C is 12.

The MV of HexDigit:::d or of HexDigit:::D is 13.

The MV of HexDigit:::e or of HexDigit:::E is 14.

The MV of HexDigit:::f or of HexDigit:::F is 15.

The MV of HexIntegerLiteral:::0xHexDigit is the MV of HexDigit.

The MV of HexIntegerLiteral:::0XHexDigit is the MV of HexDigit.

The MV of HexIntegerLiteral:::HexIntegerLiteralHexDigit is (the MV of HexIntegerLiteral times
16) plus the MV of HexDigit.

Once the exact MV for a String numeric literal has been determined, it is then rounded to a value of the Number type. If
the MV is 0, then the rounded value is +0 unless the first non white space character in the String numeric literal is
‘-’, in which case the rounded value is −0. Otherwise, the rounded value must be the Number
value for the MV (in the sense defined in 8.5), unless the literal includes a StrUnsignedDecimalLiteral and the literal has more than 20 significant digits, in which case the Number
value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th
with a 0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with
a 0 digit and then incrementing the literal at the 20th digit position. A digit is significant if it is not part of
an ExponentPart and

it is not 0; or

there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to
its right.

Let int32bit be posIntmodulo 232; that is, a finite integer value k of
Number type with positive sign and less than 232 in magnitude such that the mathematical difference of
posInt and k is mathematically an integer multiple of 232.

If int32bit is greater than or equal to 231, return int32bit − 232, otherwise
return int32bit.

NOTE Given the above definition of ToInt32:

The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves
that value unchanged.

ToInt32(ToUint32(x)) is equal to ToInt32(x) for all values of x. (It is to preserve this latter property that
+∞ and −∞ are mapped to +0.)

Let int32bit be posIntmodulo 232; that is, a finite integer value k of
Number type with positive sign and less than 232 in magnitude such that the mathematical difference of
posInt and k is mathematically an integer multiple of 232.

Let int16bit be posIntmodulo 216; that is, a finite integer value
k of Number type with positive sign and less than 216 in magnitude such that the mathematical
difference of posInt and k is mathematically an integer multiple of 216.

Return int16bit.

NOTE Given the above definition of ToUint16:

The substitution of 216 for 232 in step 4 is the only difference between ToUint32 and
ToUint16.

The abstract operation ToString converts a Number m to String format as follows:

If m is NaN, return the String "NaN".

If m is +0 or −0, return the String "0".

If m is less than zero, return the String concatenation of the String "-" and ToString(−m).

If m is infinity, return the String "Infinity".

Otherwise, let n, k, and s be integers such that k ≥ 1, 10k−1
≤ s < 10k, the Number value for s × 10n−k is
m, and k is as small as possible. Note that k is the number of digits in the decimal
representation of s, that s is not divisible by 10, and that the least significant digit of s is
not necessarily uniquely determined by these criteria.

If k ≤ n ≤ 21, return the String consisting of the k digits of the decimal representation
of s (in order, with no leading zeroes), followed by n−k occurrences of the character
‘0’.

If 0 < n ≤ 21, return the String consisting of the most significant n digits of the decimal
representation of s, followed by a decimal point ‘.’, followed by the remaining
k−n digits of the decimal representation of s.

If −6 < n ≤ 0, return the String consisting of the character ‘0’, followed by a
decimal point ‘.’, followed by −n occurrences of the character
‘0’, followed by the k digits of the decimal representation of s.

Otherwise, if k = 1, return the String consisting of the single digit of s, followed by lowercase
character ‘e’, followed by a plus sign ‘+’ or minus sign
‘−’ according to whether n−1 is positive or negative, followed by the
decimal representation of the integer abs(n−1) (with no leading zeroes).

Return the String consisting of the most significant digit of the decimal representation of s, followed by a
decimal point ‘.’, followed by the remaining k−1 digits of the decimal representation of
s, followed by the lowercase character ‘e’, followed by a plus sign
‘+’ or minus sign ‘−’ according to whether n−1 is
positive or negative, followed by the decimal representation of the integer abs(n−1) (with no leading zeroes).

NOTE 1 The following observations may be useful as guidelines for implementations, but are not
part of the normative requirements of this Standard:

If x is any Number value other than −0, then ToNumber(ToString(x)) is exactly the same Number value as x.

The least significant digit of s is not always uniquely determined by the requirements listed in step 5.

NOTE 2 For implementations that provide more accurate conversions than required by the rules
above, it is recommended that the following alternative version of step 5 be used as a guideline:

Otherwise, let n, k, and s be integers such that k ≥ 1, 10k−1
≤ s < 10k, the Number value for s × 10n−k is
m, and k is as small as possible. If there are multiple possibilities for s, choose the value of
s for which s × 10n−k is closest in value to m. If there are two
such possible values of s, choose the one that is even. Note that k is the number of digits in the decimal
representation of s and that s is not divisible by 10.

NOTE 3 Implementers of ECMAScript may find useful the paper and code written by David M. Gay
for binary-to-decimal conversion of floating-point numbers:

Global code is source text that is treated as an ECMAScript Program. The global code of a particular
Program does not include any source text that is parsed as part of a FunctionBody.

Eval code is the source text supplied to the built-in eval function. More precisely, if the
parameter to the built-in eval function is a String, it is treated as an ECMAScript Program. The eval
code for a particular invocation of eval is the global code portion of that Program.

Function code is source text that is parsed as part of a FunctionBody. The function code of a
particular FunctionBody does not include any source text that is parsed as part of a nested FunctionBody.
Function code also denotes the source text supplied when using the built-in Function object as a
constructor. More precisely, the last parameter provided to the Function constructor is converted to a String
and treated as the FunctionBody. If more than one parameter is provided to the Function constructor,
all parameters except the last one are converted to Strings and concatenated together, separated by commas. The resulting
String is interpreted as the FormalParameterList for the FunctionBody defined by the last parameter. The
function code for a particular instantiation of a Function does not include any source text that is parsed as
part of a nested FunctionBody.

An ECMAScript Program syntactic unit may be processed using either unrestricted or strict mode
syntax and semantics. When processed using strict mode the three types of ECMAScript code are referred to as strict global
code, strict eval code, and strict function code. Code is interpreted as strict mode code in the following situations:

Function code that is part of a FunctionDeclaration, FunctionExpression, or accessor PropertyAssignment is strict function code if
its FunctionDeclaration, FunctionExpression, or PropertyAssignment is contained in strict mode code or if the function code begins with a Directive Prologue that contains a Use Strict Directive.

Function code that is supplied as the last argument to the built-in Function constructor is strict function code if
the last argument is a String that when processed as a FunctionBody begins with a Directive Prologue that contains a Use Strict Directive.

A Lexical Environment is a specification type used to define the association of Identifiers
to specific variables and functions based upon the lexical nesting structure of ECMAScript code. A Lexical Environment
consists of an Environment Record and a possibly null reference to an outer Lexical
Environment. Usually a Lexical Environment is associated with some specific syntactic structure of ECMAScript code such as a
FunctionDeclaration, a WithStatement, or a Catch
clause of a TryStatement and a new Lexical Environment is created each time such code is
evaluated.

An Environment Record records the identifier bindings that are created within the scope of
its associated Lexical Environment.

The outer environment reference is used to model the logical nesting of Lexical Environment values. The outer reference of
a (inner) Lexical Environment is a reference to the Lexical Environment that logically surrounds the inner Lexical
Environment. An outer Lexical Environment may, of course, have its own outer Lexical Environment. A Lexical Environment may
serve as the outer environment for multiple inner Lexical Environments. For example, if a FunctionDeclaration contains two nested FunctionDeclarations then the Lexical
Environments of each of the nested functions will have as their outer Lexical Environment the Lexical Environment of the
current execution of the surrounding function.

Lexical Environments and Environment Record values are purely specification mechanisms and need
not correspond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to
directly access or manipulate such values.

There are two kinds of Environment Record values used in this specification: declarative environment records and
object environment records. Declarative environment records are used to define the effect of ECMAScript language
syntactic elements such as FunctionDeclarations, VariableDeclarations, and
Catch clauses that directly associate identifier bindings with ECMAScript language values. Object
environment records are used to define the effect of ECMAScript elements such as Program and WithStatement that associate identifier bindings with the properties of some object.

For specification purposes Environment Record values can be thought of as existing in a simple object-oriented hierarchy
where Environment Record is an abstract class with two concrete subclasses, declarative environment record and object
environment record. The abstract class includes the abstract specification methods defined in Table 17. These abstract
methods have distinct concrete algorithms for each of the concrete subclasses.

Table 17 — Abstract Methods of Environment Records

Method

Purpose

HasBinding(N)

Determine if an environment record has a binding for an identifier. Return true if it does and false if it does not. The String value N is the text of the identifier.

CreateMutableBinding(N, D)

Create a new mutable binding in an environment record. The String value N is the text of the bound name. If the optional Boolean argument D is true the binding is may be subsequently deleted.

SetMutableBinding(N,V, S)

Set the value of an already existing mutable binding in an environment record. The String value N is the text of the bound name. V is the value for the binding and may be a value of any ECMAScript language type. S is a Boolean flag. If S is true and the binding cannot be set throw a TypeError exception. S is used to identify strict mode references.

GetBindingValue(N,S)

Returns the value of an already existing binding from an environment record. The String value N is the text of the bound name. S is used to identify strict mode references. If S is true and the binding does not exist or is uninitialised throw a ReferenceError exception.

DeleteBinding(N)

Delete a binding from an environment record. The String value N is the text of the bound name If a binding for N exists, remove the binding and return true. If the binding exists but cannot be removed return false. If the binding does not exist return true.

ImplicitThisValue()

Returns the value to use as the this value on calls to function objects that are obtained as binding values from this environment record.

In addition to the mutable bindings supported by all Environment Records, declarative environment records also provide
for immutable bindings. An immutable binding is one where the association between an identifier and a value may not be
modified once it has been established. Creation and initialisation of immutable binding are distinct steps so it is
possible for such bindings to exist in either an initialised or uninitialised state. Declarative environment records
support the methods listed in Table 18 in addition to the Environment Record abstract specification methods:

Table 18 — Additional Methods of Declarative Environment Records

Method

Purpose

CreateImmutableBinding(N)

Create a new but uninitialised immutable binding in an environment record. The String value N is the text of the bound name.

InitializeImmutableBinding(N,V)

Set the value of an already existing but uninitialised immutable binding in an environment record. The String value N is the text of the bound name. V is the value for the binding and is a value of any ECMAScript language type.

The behaviour of the concrete specification methods for Declarative Environment Records is defined by the following
algorithms.

The concrete Environment Record method CreateMutableBinding for declarative environment
records creates a new mutable binding for the name N that is initialised to the value undefined. A
binding must not already exist in this Environment Record for N. If Boolean
argument D is provided and has the value true the new binding is marked as being subject to
deletion.

The concrete Environment Record method SetMutableBinding for declarative environment
records attempts to change the bound value of the current binding of the identifier whose name is the value of the
argument N to the value of argument V. A binding for N must already exist. If the
binding is an immutable binding, a TypeError is thrown if S is
true.

The concrete Environment Record method GetBindingValue for declarative environment records
simply returns the value of its bound identifier whose name is the value of the argument N. The binding must
already exist. If S is true and the binding is an uninitialised immutable binding throw a
ReferenceError exception.

The concrete Environment Record method CreateImmutableBinding for declarative environment
records creates a new immutable binding for the name N that is initialised to the value undefined. A
binding must not already exist in this environment record for N.

The concrete Environment Record method InitializeImmutableBinding for declarative
environment records is used to set the bound value of the current binding of the identifier whose name is the value of
the argument N to the value of argument V. An uninitialised immutable binding for N
must already exist.

Each object environment record is associated with an object called its binding object.
An object environment record binds the set of identifier names that directly correspond to the
property names of its binding object. Property names that are not an IdentifierName are not
included in the set of bound identifiers. Both own and inherited properties are included in the set regardless of the
setting of their [[Enumerable]] attribute. Because properties can be dynamically added and deleted from objects, the set
of identifiers bound by an object environment record may potentially change as a side-effect of
any operation that adds or deletes properties. Any bindings that are created as a result of such a side-effect are
considered to be a mutable binding even if the Writable attribute of the corresponding property has the value
false. Immutable bindings do not exist for object environment records.

Object environment records can be configured to provide their binding object as an implicit this value for use in
function calls. This capability is used to specify the behaviour of With Statement (12.10)
induced bindings. The capability is controlled by a provideThis Boolean value that is associated with each object environment record. By default, the value of provideThis is false for any
object environment record.

The behaviour of the concrete specification methods for Object Environment Records is defined by the following
algorithms.

The concrete Environment Record method CreateMutableBinding for object environment records
creates in an environment record’s associated binding object a property whose name is the String value and
initialises it to the value undefined. A property named N must not already exist in the binding
object. If Boolean argument D is provided and has the value true the new property’s
[[Configurable]] attribute is set to true, otherwise it is set to false.

The concrete Environment Record method SetMutableBinding for object environment records
attempts to set the value of the environment record’s associated binding object’s property whose name is the
value of the argument N to the value of argument V. A property named N should already
exist but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument
S.

The concrete Environment Record method GetBindingValue for object environment records
returns the value of its associated binding object’s property whose name is the String value of the argument
identifier N. The property should already exist but if it does not the result depends upon the value of the
S argument:

The concrete Environment Record method DeleteBinding for object environment records can
only delete bindings that correspond to properties of the environment object whose [[Configurable]] attribute have the
value true.

The abstract operation GetIdentifierReference is called with a Lexical Environmentlex, an identifier String name, and a Boolean flag strict. The value of lex
may be null. When called, the following steps are performed:

If lex is the value null, then

Return a value of type Reference whose base value is undefined, whose referenced
name is name, and whose strict mode flag is strict.

Let envRec be lex’s environment record.

Let exists be the result of calling the HasBinding(N) concrete method of envRec passing
name as the argument N.

If exists is true, then

Return a value of type Reference whose base value is envRec, whose referenced name
is name, and whose strict mode flag is strict.

When control is transferred to ECMAScript executable code, control is entering an execution context. Active
execution contexts logically form a stack. The top execution context on this logical stack is the running execution context. A
new execution context is created whenever control is transferred from the executable code associated with the currently
running execution context to executable code that is not associated with that execution context. The newly created execution
context is pushed onto the stack and becomes the running execution context.

An execution context contains whatever state is necessary to track the execution progress of its associated code. In
addition, each execution context has the state components listed in Table 19.

Table 19 —Execution Context State Components

Component

Purpose

LexicalEnvironment

Identifies the Lexical Environment used to resolve identifier references made by code within this execution context.

VariableEnvironment

Identifies the Lexical Environment whose environment record holds bindings created by VariableStatements and FunctionDeclarations within this execution context.

ThisBinding

The value associated with the this keyword within ECMAScript code associated with this execution context.

The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical Environments. When an
execution context is created its LexicalEnvironment and VariableEnvironment components initially have the same value. The
value of the VariableEnvironment component never changes while the value of the LexicalEnvironment component may change during
execution of code within an execution context.

In most situations only the running execution context (the top of the execution context stack) is directly manipulated by
algorithms within this specification. Hence when the terms “LexicalEnvironment”,
“VariableEnvironment” and “ThisBinding” are used without qualification they are in reference to those
components of the running execution context.

An execution context is purely a specification mechanism and need not correspond to any particular artefact of an
ECMAScript implementation. It is impossible for an ECMAScript program to access an execution context.

Identifier resolution is the process of determining the binding of an Identifier using the LexicalEnvironment of the running execution context. During execution of ECMAScript code, the syntactic
production PrimaryExpression:Identifier is evaluated using the following algorithm:

Evaluation of global code or code using the eval function (15.1.2.1) establishes and enters a
new execution context. Every invocation of an ECMAScript code function (13.2.1) also establishes and
enters a new execution context, even if a function is calling itself recursively. Every return exits an execution context. A
thrown exception may also exit one or more execution contexts.

The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that
invoked the eval if either the code of the calling context or the eval code is strict code.
Instead such bindings are instantiated in a new VariableEnvironment that is only accessible to the
eval code.

Which Environment Record is used to bind a declaration and its kind depends upon the type of
ECMAScript code executed by the execution context, but the remainder of the behaviour is generic. On entering an execution
context, bindings are created in the VariableEnvironment as follows using the caller provided
code and, if it is function code, argument Listargs:

Let env be the environment record component of the running execution context’s VariableEnvironment.

If code is eval code, then let configurableBindings be true else let configurableBindings be
false.

When control enters an execution context for function code, an arguments object is created unless (as specified in 10.5) the identifier arguments occurs as an Identifier in the
function’s FormalParameterList or occurs as the Identifier of a VariableDeclaration or FunctionDeclaration contained in the function code.

The arguments object is created by calling the abstract operation CreateArgumentsObject with arguments func the
function object whose code is to be evaluated, names a List containing the function’s
formal parameter names, args the actual arguments passed to the [[Call]] internal method, env the
variable environment for the function code, and strict a Boolean that indicates whether or not the function code is
strict code. When CreateArgumentsObject is called the following steps are performed:

The abstract operation MakeArgGetter called with String name and environment record env
creates a function object that when executed returns the value bound for name in env. It performs the
following steps:

Let body be the result of concatenating the Strings "return ", name, and
";".

Return the result of creating a function object as described in 13.2 using no
FormalParameterList, body for FunctionBody, env as Scope, and true for
Strict.

The abstract operation MakeArgSetter called with String name and environment record env
creates a function object that when executed sets the value bound for name in env. It performs the
following steps:

Let param be the String name concatenated with the String "_arg".

Let body be the String "<name> = <param>;" with <name> replaced by the value of name and
<param> replaced by the value of param.

Return the result of creating a function object as described in 13.2 using a List containing the single String param as FormalParameterList, body for
FunctionBody, env as Scope, and true for Strict.

The [[Get]] internal method of an arguments object for a non-strict mode function with formal parameters when called with a
property name P performs the following steps:

Let map be the value of the [[ParameterMap]] internal property of the arguments object.

Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as
the argument.

If the value of isMapped is undefined, then

Let v be the result of calling the default [[Get]] internal method (8.12.3) on the
arguments object passing P as the argument.

If P is "caller" and v is a strict mode Function object, throw a TypeError
exception.

Return v.

Else, map contains a formal parameter mapping for P so,

Return the result of calling the [[Get]] internal method of map passing P as the argument.

The [[GetOwnProperty]] internal method of an arguments object for a non-strict mode function with formal parameters when
called with a property name P performs the following steps:

Let desc be the result of calling the default [[GetOwnProperty]] internal method (8.12.1) on the arguments object passing P as the argument.

If desc is undefined then return desc.

Let map be the value of the [[ParameterMap]] internal property of the arguments object.

Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as
the argument.

If the value of isMapped is not undefined, then

Set desc.[[Value]] to the result of calling the [[Get]] internal method of map passing P as the
argument.

Return desc.

The [[DefineOwnProperty]] internal method of an arguments object for a non-strict mode function with formal parameters
when called with a property name P, Property DescriptorDesc, and
Boolean flag Throw performs the following steps:

Let map be the value of the [[ParameterMap]] internal property of the arguments object.

Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as
the argument.

Let allowed be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on the arguments object passing P, Desc, and false as the
arguments.

If allowed is false, then

If Throw is true then throw a TypeError exception, otherwise return false.

Call the [[Delete]] internal method of map passing P, and false as the arguments.

Else

If Desc.[[Value]] is present, then

Call the [[Put]] internal method of map passing P, Desc.[[Value]], and Throw as
the arguments.

If Desc.[[Writable]] is present and its value is false, then

Call the [[Delete]] internal method of map passing P and false as arguments.

Return true.

The [[Delete]] internal method of an arguments object for a non-strict mode function with formal parameters when called
with a property name P and Boolean flag Throw performs the following steps:

Let map be the value of the [[ParameterMap]] internal property of the arguments object.

Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as
the argument.

Let result be the result of calling the default [[Delete]] internal method (8.12.7) on
the arguments object passing P and Throw as the arguments.

If result is true and the value of isMapped is not undefined, then

Call the [[Delete]] internal method of map passing P, and false as the arguments.

Return result.

NOTE 1 For non-strict mode functions the array index (defined in 15.4)
named data properties of an arguments object whose numeric name values are less than the number of formal parameters of the
corresponding function object initially share their values with the corresponding argument bindings in the function’s
execution context. This means that changing the property changes the corresponding value of the argument binding and
vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed
into an accessor property. For strict mode functions, the values of the arguments object’s properties are simply a
copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal
parameter values.

NOTE 2 The ParameterMap object and its property values are used as a device for specifying the
arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its
properties are not directly accessible from ECMAScript code. An ECMAScript implementation does not need to actually create
or use such objects to implement the specified semantics.

NOTE 3 Arguments objects for strict mode functions define non-configurable accessor properties
named "caller" and "callee" which throw a TypeError exception on access. The
"callee" property has a more specific meaning for non-strict mode functions and a "caller"
property has historically been provided as an implementation-defined extension by some ECMAScript implementations. The
strict mode definition of these properties exists to ensure that neither of them is defined in any other manner by
conforming ECMAScript implementations.

An array initialiser is an expression describing the initialisation of an Array object, written in a form of a literal.
It is a list of zero or more expressions, each of which represents an array element, enclosed in square brackets. The
elements need not be literals; they are evaluated each time the array initialiser is evaluated.

Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is
not preceded by an AssignmentExpression (i.e., a comma at the beginning or after another comma), the
missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array
elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of
the Array.

Syntax

ArrayLiteral:

[Elisionopt]

[ElementList]

[ElementList,Elisionopt]

ElementList:

ElisionoptAssignmentExpression

ElementList,ElisionoptAssignmentExpression

Elision:

,

Elision,

Semantics

The production ArrayLiteral:[Elisionopt] is evaluated as
follows:

Let array be the result of creating a new object as if by the expression new Array() where
Array is the standard built-in constructor with that name.

Let pad be the result of evaluating Elision; if not present, use the numeric value zero.

Call the [[Put]] internal method of array with arguments "length", pad, and false.

Return array.

The production ArrayLiteral:[ElementList] is evaluated as follows:

Return the result of evaluating ElementList.

The production ArrayLiteral:[ElementList,Elisionopt] is evaluated as follows:

Let array be the result of evaluating ElementList.

Let pad be the result of evaluating Elision; if not present, use the numeric value zero.

Let len be the result of calling the [[Get]] internal method of array with argument "length".

Call the [[Put]] internal method of array with arguments "length", ToUint32(pad+len), and false.

Return array.

The production ElementList:ElisionoptAssignmentExpression is evaluated as follows:

Let array be the result of creating a new object as if by the expression new Array() where
Array is the standard built-in constructor with that name.

Let firstIndex be the result of evaluating Elision; if not present, use the numeric value zero.

NOTE [[DefineOwnProperty]] is used to ensure that own properties are defined for the array
even if the standard built-in Array prototype object has been modified in a manner that would preclude the creation of new
own properties using [[Put]].

An object initialiser is an expression describing the initialisation of an Object, written in a form resembling a
literal. It is a list of zero or more pairs of property names and associated values, enclosed in curly braces. The values
need not be literals; they are evaluated each time the object initialiser is evaluated.

Syntax

ObjectLiteral:

{}

{PropertyNameAndValueList}

{PropertyNameAndValueList,}

PropertyNameAndValueList:

PropertyAssignment

PropertyNameAndValueList,PropertyAssignment

PropertyAssignment:

PropertyName:AssignmentExpression

getPropertyName(){FunctionBody}

setPropertyName(PropertySetParameterList){FunctionBody}

PropertyName:

IdentifierName

StringLiteral

NumericLiteral

PropertySetParameterList:

Identifier

Semantics

The production ObjectLiteral:{} is evaluated as follows:

Return a new object created as if by the expression new Object() where Object is the
standard built-in constructor with that name.

The productions ObjectLiteral:{PropertyNameAndValueList} andObjectLiteral:{PropertyNameAndValueList,} are evaluated as
follows:

Return the result of evaluating PropertyNameAndValueList.

The production PropertyNameAndValueList:PropertyAssignment is evaluated as follows:

Let obj be the result of creating a new object as if by the expression new Object() where
Object is the standard built-in constructor with that name.

Let propId be the result of evaluating PropertyAssignment.

Call the [[DefineOwnProperty]] internal method of obj with arguments propId.name,
propId.descriptor, and false.

Return obj.

The productionPropertyNameAndValueList:PropertyNameAndValueList,PropertyAssignmentis evaluated as follows:

Let obj be the result of evaluating PropertyNameAndValueList.

Let propId be the result of evaluating PropertyAssignment.

Let previous be the result of calling the [[GetOwnProperty]] internal method of obj with argument
propId.name.

If previous is not undefined then throw a SyntaxError exception if any of the following
conditions are true

IsAccessorDescriptor(previous) is true and IsAccessorDescriptor(propId.descriptor) is true and either both
previous and propId.descriptor have [[Get]] fields or both previous and
propId.descriptor have [[Set]] fields

Call the [[DefineOwnProperty]] internal method of obj with arguments propId.name,
propId.descriptor, and false.

Return obj.

If the above steps would throw a SyntaxError then an implementation must treat the error as an early error (Clause 16).

The production PropertyAssignment:PropertyName:AssignmentExpression is evaluated as
follows:

The production PropertyAssignment:getPropertyName(){FunctionBody} is evaluated as follows:

Let propName be the result of evaluating PropertyName.

Let closure be the result of creating a new Function object as specified in 13.2 with
an empty parameter list and body specified by FunctionBody. Pass in the LexicalEnvironment of the running execution context as the Scope. Pass in true as
the Strict flag if the PropertyAssignment is contained in strict code or if
its FunctionBody is strict code.

The production PropertyAssignment:setPropertyName(PropertySetParameterList){FunctionBody} is evaluated as follows:

Let propName be the result of evaluating PropertyName.

Let closure be the result of creating a new Function object as specified in 13.2 with
parameters specified by PropertySetParameterList and body specified by FunctionBody. Pass in the LexicalEnvironment of the running execution context as the Scope. Pass in true as
the Strict flag if the PropertyAssignment is contained in strict code or if
its FunctionBody is strict code.

It is a SyntaxError if the Identifier"eval" or the Identifier"arguments" occurs as the Identifier in a PropertySetParameterList of a PropertyAssignment that is contained in strict code or if its FunctionBody is strict code.

The production PropertyName:IdentifierName is evaluated as follows:

Return the String value containing the same sequence of characters as the IdentifierName.

The production PrimaryExpression:(Expression) is evaluated as follows:

Return the result of evaluating Expression. This may be of type Reference.

NOTE This algorithm does not apply GetValue to the result of
evaluating Expression. The principal motivation for this is so that operators such as delete and
typeof may be applied to parenthesised expressions.

Return the result of calling the [[Call]] internal method on func, providing thisValue as the
this value and providing the list argList as the argument values.

The production CallExpression:CallExpressionArguments is evaluated in exactly the same manner, except
that the contained CallExpression is evaluated in step 1.

NOTE The returned result will never be of type Reference if
func is a native ECMAScript object. Whether calling a host object can return a value of type Reference is implementation-dependent. If a value of type Reference is
returned, it must be a non-strict Property Reference.

Return the result of calling the DeleteBinding concrete method of bindings, providing GetReferencedName(ref) as the argument.

NOTE When a delete operator occurs within strict mode
code, a SyntaxError exception is thrown if its UnaryExpression is a direct reference to
a variable, function argument, or function name. In addition, if a delete operator occurs within strict mode code and the property to be deleted has the attribute { [[Configurable]]: false
}, a TypeError exception is thrown.

The * operator performs multiplication, producing the product of its operands. Multiplication is
commutative. Multiplication is not always associative in ECMAScript, because of finite precision.

The result of a floating-point multiplication is governed by the rules of IEEE 754 binary double-precision
arithmetic:

If either operand is NaN, the result is NaN.

The sign of the result is positive if both operands have the same sign, negative if the operands have different
signs.

Multiplication of an infinity by a zero results in NaN.

Multiplication of an infinity by an infinity results in an infinity. The sign is determined by the rule already
stated above.

Multiplication of an infinity by a finite nonzero value results in a signed infinity. The sign is determined by the
rule already stated above.

In the remaining cases, where neither an infinity or NaN is involved, the product is computed and rounded to the
nearest representable value using IEEE 754 round-to-nearest mode. If the magnitude is too large to represent, the result
is then an infinity of appropriate sign. If the magnitude is too small to represent, the result is then a zero of
appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754.

The / operator performs division, producing the quotient of its operands. The left operand is the dividend
and the right operand is the divisor. ECMAScript does not perform integer division. The operands and result of all division
operations are double-precision floating-point numbers. The result of division is determined by the specification of IEEE
754 arithmetic:

If either operand is NaN, the result is NaN.

The sign of the result is positive if both operands have the same sign, negative if the operands have different
signs.

Division of an infinity by an infinity results in NaN.

Division of an infinity by a zero results in an infinity. The sign is determined by the rule already stated
above.

Division of an infinity by a nonzero finite value results in a signed infinity. The sign is determined by the rule
already stated above.

Division of a finite value by an infinity results in zero. The sign is determined by the rule already stated
above.

Division of a zero by a zero results in NaN; division of zero by any other finite value results in zero, with
the sign determined by the rule already stated above.

Division of a nonzero finite value by a zero results in a signed infinity. The sign is determined by the rule already
stated above.

In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the quotient is computed
and rounded to the nearest representable value using IEEE 754 round-to-nearest mode. If the magnitude is too large to
represent, the operation overflows; the result is then an infinity of appropriate sign. If the magnitude is too small to
represent, the operation underflows and the result is a zero of the appropriate sign. The ECMAScript language requires
support of gradual underflow as defined by IEEE 754.

The % operator yields the remainder of its operands from an implied division; the left operand is the
dividend and the right operand is the divisor.

NOTE In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it
also accepts floating-point operands.

The result of a floating-point remainder operation as computed by the % operator is not the same as the
“remainder” operation defined by IEEE 754. The IEEE 754 “remainder” operation computes the remainder
from a rounding division, not a truncating division, and so its behaviour is not analogous to that of the usual integer
remainder operator. Instead the ECMAScript language defines % on floating-point operations to behave in a
manner analogous to that of the Java integer remainder operator; this may be compared with the C library function fmod.

The result of an ECMAScript floating-point remainder operation is determined by the rules of IEEE arithmetic:

If either operand is NaN, the result is NaN.

The sign of the result equals the sign of the dividend.

If the dividend is an infinity, or the divisor is a zero, or both, the result is NaN.

If the dividend is finite and the divisor is an infinity, the result equals the dividend.

If the dividend is a zero and the divisor is nonzero and finite, the result is the same as the dividend.

In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the floating-point
remainder r from a dividend n and a divisor d is defined by the mathematical relation r = n − (d × q)
where q is an integer that is negative only if n/d is negative and positive only if n/d is positive, and whose
magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d. r is
computed and rounded to the nearest representable value using IEEE 754 round-to-nearest mode.

Return the String that is the result of concatenating ToString(lprim) followed by ToString(rprim)

Return the result of applying the addition operation to ToNumber(lprim) and ToNumber(rprim). See the Note below 11.6.3.

NOTE 1 No hint is provided in the calls to ToPrimitive in steps 5 and 6.
All native ECMAScript objects except Date objects handle the absence of a hint as if the hint Number were given; Date
objects handle the absence of a hint as if the hint String were given. Host objects may handle the absence of a hint in
some other manner.

NOTE 2 Step 7 differs from step 3 of the comparison algorithm for the relational operators (11.8.5), by using the logical-or operation instead of the logical-and operation.

The + operator performs addition when applied to two operands of numeric type, producing the sum of the
operands. The - operator performs subtraction, producing the difference of two numeric operands.

Addition is a commutative operation, but not always associative.

The result of an addition is determined using the rules of IEEE 754 binary double-precision arithmetic:

If either operand is NaN, the result is NaN.

The sum of two infinities of opposite sign is NaN.

The sum of two infinities of the same sign is the infinity of that sign.

The sum of an infinity and a finite value is equal to the infinite operand.

The sum of two negative zeroes is −0. The sum of two positive zeroes, or of two zeroes of opposite sign,
is +0.

The sum of a zero and a nonzero finite value is equal to the nonzero operand.

The sum of two nonzero finite values of the same magnitude and opposite sign is +0.

In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, and the operands have the same
sign or have different magnitudes, the sum is computed and rounded to the nearest representable value using IEEE 754
round-to-nearest mode. If the magnitude is too large to represent, the operation overflows and the result is then an
infinity of appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754.

The - operator performs subtraction when applied to two operands of numeric type, producing the difference
of its operands; the left operand is the minuend and the right operand is the subtrahend. Given numeric operands
a and b, it is always the case that a–b produces the same
result as a+(–b).

NOTE The “NoIn” variants are needed to avoid confusing the in operator in a relational expression with the in operator in
a for statement.

Semantics

The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by the
operator holds between its two operands.

The RelationalExpressionNoIn productions are evaluated in the same manner as the RelationalExpression productions except that the contained RelationalExpressionNoIn
is evaluated instead of the contained RelationalExpression.

The comparison x < y, where x and y are values, produces true,
false, or undefined (which indicates that at least one operand is NaN). In addition to x and
y the algorithm takes a Boolean flag named LeftFirst as a parameter. The flag is used to
control the order in which operations with potentially visible side-effects are performed upon x and
y. It is necessary because ECMAScript specifies left to right evaluation of expressions. The default value of
LeftFirst is true and indicates that the x parameter corresponds to an expression
that occurs to the left of the y parameter’s corresponding expression. If LeftFirst
is false, the reverse is the case and operations must be performed upon y before x. Such a
comparison is performed as follows:

If the mathematical value of nx is less than the mathematical value of ny —note that these
mathematical values are both finite and not both zero—return true. Otherwise, return
false.

Else, both px and py are Strings

If py is a prefix of px, return false. (A String value p is a prefix of String value
q if q can be the result of concatenating p and some other String r. Note that any
String is a prefix of itself, because r may be the empty String.)

If px is a prefix of py, return true.

Let k be the smallest nonnegative integer such that the character at position k within px is
different from the character at position k within py. (There must be such a k, for neither
String is a prefix of the other.)

Let m be the integer that is the code unit value for the character at position k within
px.

Let n be the integer that is the code unit value for the character at position k within
py.

If m < n, return true. Otherwise, return false.

NOTE 1 Step 3 differs from step 7 in the algorithm for the addition operator + (11.6.1) in using and instead of or.

NOTE 2 The comparison of Strings uses a simple lexicographic ordering on sequences of code unit
values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and
collating order defined in the Unicode specification. Therefore String values that are canonically equal according to the
Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalised form.
Also, note that for strings containing supplementary characters, lexicographic ordering on sequences of UTF-16 code unit
values differs from that on sequences of code point values.

Semantics

The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by the
operator holds between its two operands.

The EqualityExpressionNoIn productions are evaluated in the same manner as the EqualityExpression productions except that the contained EqualityExpressionNoIn and
RelationalExpressionNoIn are evaluated instead of the contained EqualityExpression and RelationalExpression, respectively.

If Type(x) is String, then return true if x and y are exactly the
same sequence of characters (same length and same characters in corresponding positions). Otherwise, return
false.

If Type(x) is Boolean, return true if x and y are both
true or both false. Otherwise, return false.

Return true if x and y refer to the same object. Otherwise, return false.

If x is null and y is undefined, return true.

If x is undefined and y is null, return true.

If Type(x) is Number and Type(y) is String,return the
result of the comparison x == ToNumber(y).

If Type(x) is String and Type(y) is Number,return the
result of the comparison ToNumber(x) == y.

If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.

If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).

If Type(x) is either String or Number and Type(y) is
Object,return the result of the comparison x == ToPrimitive(y).

If Type(x) is Object and Type(y) is either String or
Number,return the result of the comparison ToPrimitive(x) == y.

Return false.

NOTE 1 Given the above definition of equality:

String comparison can be forced by: "" + a == "" + b.

Numeric comparison can be forced by: +a == +b.

Boolean comparison can be forced by: !a == !b.

NOTE 2 The equality operators maintain the following invariants:

A!=B is equivalent to !(A==B).

A==B is equivalent to B==A, except
in the order of evaluation of A and B.

NOTE 3 The equality operator is not always transitive. For example, there might be two distinct
String objects, each representing the same String value; each String object would be considered equal to the String value
by the == operator, but the two String objects would not be equal to each other. For Example:

new String("a")=="a" and "a"==new
String("a")are both true.

new String("a")==new String("a") is false.

NOTE 4 Comparison of Strings uses a simple equality test on sequences of code unit values.
There is no attempt to use the more complex, semantically oriented definitions of character or string equality and
collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the
Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalised
form.

The LogicalANDExpressionNoIn and LogicalORExpressionNoIn productions are
evaluated in the same manner as the LogicalANDExpression and LogicalORExpression productions except that the contained LogicalANDExpressionNoIn,
BitwiseORExpressionNoIn and LogicalORExpressionNoIn are evaluated instead of
the contained LogicalANDExpression, BitwiseORExpression and LogicalORExpression, respectively.

NOTE The value produced by a && or || operator is not
necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.

The ConditionalExpressionNoIn production is evaluated in the same manner as the ConditionalExpression production except that the contained LogicalORExpressionNoIn,
AssignmentExpression and AssignmentExpressionNoIn are evaluated instead of the
contained LogicalORExpression, first AssignmentExpression and second AssignmentExpression, respectively.

NOTE The grammar for a ConditionalExpression in ECMAScript is a little bit different from that in
C and Java, which each allow the second subexpression to be an Expression but restrict the third expression to be a
ConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed
by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre
expression.

Semantics

The AssignmentExpressionNoIn productions are evaluated in the same manner as the AssignmentExpression productions except that the contained ConditionalExpressionNoIn
and AssignmentExpressionNoIn are evaluated instead of the contained ConditionalExpression and AssignmentExpression, respectively.

NOTE When an assignment occurs within strict mode code, its LeftHandSide must not evaluate to an unresolvable reference. If it does a ReferenceError
exception is thrown upon assignment. The LeftHandSide also may not be a reference to a data
property with the attribute value {[[Writable]]:false}, to an
accessor property with the attribute value {[[Set]]:undefined},
nor to a non-existent property of an object whose [[Extensible]] internal property has the value false. In these
cases a TypeError exception is thrown.

The production AssignmentExpression:LeftHandSideExpressionAssignmentOperatorAssignmentExpression , where AssignmentOperator is @= and @
represents one of the operators indicated above, is evaluated as follows:

The ExpressionNoIn production is evaluated in the same manner as the Expression production except that the contained ExpressionNoIn and AssignmentExpressionNoIn are evaluated instead of the contained Expression and AssignmentExpression, respectively.

NOTEGetValue must be called even though its value is not used because
it may have observable side-effects.

Semantics

A Statement can be part of a LabelledStatement, which itself can be part of a
LabelledStatement, and so on. The labels introduced this way are collectively referred to as the
“current label set” when describing the semantics of individual statements. A LabelledStatement has no semantic meaning other than the introduction of a label to a label set. The
label set of an IterationStatement or a SwitchStatement initially contains the
single element empty. The label set of any other statement is initially empty.

NOTE Several widely used implementations of ECMAScript are known to support the use of FunctionDeclaration as a Statement. However there are significant and irreconcilable variations among the implementations in the
semantics applied to such FunctionDeclarations. Because of these irreconcilable differences, the use
of a FunctionDeclaration as a Statement results in code that is not reliably
portable among implementations. It is recommended that ECMAScript implementations either disallow this usage of FunctionDeclaration or issue a warning when such a usage is encountered. Future editions of ECMAScript may
define alternative portable means for declaring functions in a Statement context.

If an exception was thrown, return (throw, V, empty) where V is the exception. (Execution now proceeds as if no
exception were thrown.)

If s.value is empty, let V = sl.value, otherwise let
V = s.value.

Return (s.type, V, s.target).

NOTE Steps 5 and 6 of the above algoritm ensure that the value of a StatementList is the value of the last value producing Statement in the StatementList. For example, the following calls to the eval function all return the value
1:

A variable statement declares variables that are created as defined in 10.5. Variables are
initialised to undefined when created. A variable with an Initialiser is assigned the value of
its AssignmentExpression when the VariableStatement is executed, not when the
variable is created.

Semantics

The production VariableStatement:varVariableDeclarationList; is evaluated as
follows:

Evaluate VariableDeclarationList.

Return (normal, empty, empty).

The production VariableDeclarationList:VariableDeclaration is evaluated as follows:

Evaluate VariableDeclaration.

The production VariableDeclarationList:VariableDeclarationList,VariableDeclaration is
evaluated as follows:

Evaluate VariableDeclarationList.

Evaluate VariableDeclaration.

The production VariableDeclaration:Identifier is evaluated as follows:

Return a String value containing the same sequence of characters as in the Identifier.

The production VariableDeclaration:IdentifierInitialiser is evaluated as follows:

Let lhs be the result of evaluating Identifier as described in 11.1.2.

Return a String value containing the same sequence of characters as in the Identifier.

NOTE The String value of a VariableDeclaration is used in the evaluation
of for-in statements (12.6.4).

If a VariableDeclaration is nested within a with statement and the Identifier in the VariableDeclaration is the same as a property name of the binding object of the with statement’s object environment record, then step 4 will assign value to the property instead of to the VariableEnvironment binding of the Identifier.

The production Initialiser:=AssignmentExpression is evaluated as follows:

Return the result of evaluating AssignmentExpression.

The VariableDeclarationListNoIn, VariableDeclarationNoIn and InitialiserNoIn productions are evaluated in the same manner as the VariableDeclarationList, VariableDeclaration and Initialiser
productions except that the contained VariableDeclarationListNoIn, VariableDeclarationNoIn, InitialiserNoIn and AssignmentExpressionNoIn are evaluated instead of the contained VariableDeclarationList, VariableDeclaration, Initialiser
and AssignmentExpression, respectively.

Syntax

Semantics

Syntax

ExpressionStatement:

[lookahead ∉ {{, function}]Expression;

NOTE An ExpressionStatement cannot start with an opening curly brace
because that might make it ambiguous with a Block. Also, an ExpressionStatement cannot start with the function keyword because that might make it
ambiguous with a FunctionDeclaration.

Semantics

The production ExpressionStatement:[lookahead ∉ {{, function}]Expression; is evaluated as follows:

The mechanics and order of enumerating the properties (step 6.a in the first algorithm, step 7.a in the second) is not
specified. Properties of the object being enumerated may be deleted during enumeration. If a property that has not yet been
visited during enumeration is deleted, then it will not be visited. If new properties are added to the object being
enumerated during enumeration, the newly added properties are not guaranteed to be visited in the active enumeration. A
property name must not be visited more than once in any enumeration.

Enumerating the properties of an object includes enumerating properties of its prototype, and the prototype of the
prototype, and so on, recursively; but a property of a prototype is not enumerated if it is “shadowed” because
some previous object in the prototype chain has a property with the same name. The values of [[Enumerable]] attributes are
not considered when determining if a property of a prototype object is shadowed by a previous object on the prototype
chain.

Syntax

BreakStatement:

break;

break[no LineTerminator here]Identifier;

Semantics

A program is considered syntactically incorrect if either of the following is true:

The program contains a break statement without the optional Identifier, which is not nested,
directly or indirectly (but not crossing function boundaries), within an IterationStatement or a
SwitchStatement.

The program contains a break statement with the optional Identifier, where Identifier does
not appear in the label set of an enclosing (but not crossing function boundaries) Statement.

Syntax

ReturnStatement:

return;

return[no LineTerminator here]Expression;

Semantics

An ECMAScript program is considered syntactically incorrect if it contains a return statement that is not
within a FunctionBody. A return statement causes a function to cease execution and return
a value to the caller. If Expression is omitted, the return value is undefined. Otherwise, the
return value is the value of Expression.

Let C be the result of evaluating Statement but if an exception is thrown during the evaluation, let
C be (throw, V, empty), where V is the exception. (Execution now proceeds as if no exception were
thrown.)

NOTE Evaluating CaseClause does not execute the associated StatementList. It simply evaluates the Expression and returns the value, which the
CaseBlock algorithm uses to determine which StatementList to start
executing.

Syntax

Semantics

A Statement may be prefixed by a label. Labelled statements are only used in conjunction with
labelled break and continue statements. ECMAScript has no goto statement.

An ECMAScript program is considered syntactically incorrect if it contains a LabelledStatement that
is enclosed by a LabelledStatement with the same Identifier as label. This
does not apply to labels appearing within the body of a FunctionDeclaration that is nested, directly
or indirectly, within a labelled statement.

The production Identifier:Statement is evaluated by adding Identifier to the label set of Statement and then evaluating Statement. If the LabelledStatement itself has a non-empty label set, these labels are also added to the label set of Statement before evaluating it. If the result of evaluating Statement is
(break, V, L) where L is equal to Identifier, the production
results in (normal, V, empty).

Prior to the evaluation of a LabelledStatement, the contained Statement is
regarded as possessing an empty label set, unless it is an IterationStatement or a SwitchStatement, in which case it is regarded as possessing a label set consisting of the single element,
empty.

Syntax

TryStatement:

tryBlockCatch

tryBlockFinally

tryBlockCatchFinally

Catch:

catch(Identifier)Block

Finally:

finallyBlock

The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime
error or a throw statement. The catch clause provides the exception-handling code. When a catch
clause catches an exception, its Identifier is bound to that exception.

Semantics

The production TryStatement:tryBlockCatch is evaluated as follows:

Let B be the result of evaluating Block.

If B.type is not throw, return B.

Return the result of evaluating Catch with parameter B.value.

The production TryStatement:tryBlockFinally is evaluated as follows:

Let B be the result of evaluating Block.

Let F be the result of evaluating Finally.

If F.type is normal, return B.

Return F.

The production TryStatement:tryBlockCatchFinally is
evaluated as follows:

Semantics

The productionFunctionDeclaration:functionIdentifier(FormalParameterListopt){FunctionBody}is instantiated as follows during Declaration Binding
instantiation (10.5):

Return the result of creating a new Function object as specified in 13.2 with parameters specified
by FormalParameterListopt, and body specified by FunctionBody. Pass in the VariableEnvironment of the running execution context as the Scope. Pass in true as the
Strict flag if the FunctionDeclaration is contained in strict code or if its
FunctionBody is strict code.

The productionFunctionExpression:function(FormalParameterListopt){FunctionBody}is evaluated
as follows:

Return the result of creating a new Function object as specified in 13.2 with parameters specified
by FormalParameterListopt and body specified by FunctionBody. Pass in the LexicalEnvironment of the running execution context as the Scope. Pass in true as the
Strict flag if the FunctionExpression is contained in strict code or if its
FunctionBody is strict code.

The productionFunctionExpression:functionIdentifier(FormalParameterListopt){FunctionBody}is evaluated as follows:

Call the CreateImmutableBinding concrete method of envRec passing the String value of Identifier as the
argument.

Let closure be the result of creating a new Function object as specified in 13.2 with
parameters specified by FormalParameterListopt and body specified by FunctionBody. Pass in
funcEnv as the Scope. Pass in true as the Strict flag if the FunctionExpression is
contained in strict code or if its FunctionBody is strict
code.

Call the InitializeImmutableBinding concrete method of envRec passing the String value of Identifier and
closure as the arguments.

Return closure.

NOTE The Identifier in a FunctionExpression can be referenced from inside
the FunctionExpression'sFunctionBody to allow the function to call itself recursively.
However, unlike in a FunctionDeclaration, the Identifier in a FunctionExpression cannot be referenced from and does not affect the scope enclosing the FunctionExpression.

The production FunctionBody:SourceElementsopt is evaluated as follows:

Given an optional parameter list specified by FormalParameterList, a body specified by FunctionBody, a Lexical Environment specified by Scope, and
a Boolean flag Strict, a Function object is constructed as follows:

Create a new native ECMAScript object and let F be that object.

Set all the internal methods, except for [[Get]], of F as described in 8.12.

Set the [[Class]] internal property of F to "Function".

Set the [[Prototype]] internal property of F to the standard built-in Function prototype object as specified in
15.3.3.1.

When the [[Call]] internal method for a Function object F is called with a this value and a list of arguments,
the following steps are taken:

Let funcCtx be the result of establishing a new execution context for function code using the value of
F's [[FormalParameters]] internal property, the passed arguments Listargs, and
the this value as described in 10.4.3.

Let result be the result of evaluating the FunctionBody that is the value of F's [[Code]]
internal property. If F does not have a [[Code]] internal property or if its value is an empty
FunctionBody, then result is (normal, undefined, empty).

A Directive Prologue is the longest sequence of ExpressionStatement productions occurring as the
initial SourceElement productions of a Program or FunctionBody and where each ExpressionStatement in the sequence consists entirely of
a StringLiteral token followed a semicolon. The
semicolon may appear explicitly or may be inserted by automatic semicolon insertion. A Directive
Prologue may be an empty sequence.

A Use Strict Directive is an ExpressionStatement in a Directive Prologue whose StringLiteral is either the exact character sequences "usestrict" or
'usestrict'. A Use Strict Directive may not contain an EscapeSequence
or LineContinuation.

A Directive Prologue may contain more than one Use Strict Directive. However, an implementation may issue a warning if this
occurs.

NOTE The ExpressionStatement productions of a Directive Prologue are
evaluated normally during evaluation of the containing SourceElements production. Implementations
may define implementation specific meanings for ExpressionStatement productions which are not a Use
Strict Directive and which occur in a Directive Prologue. If an appropriate notification mechanism exists, an
implementation should issue a warning if it encounters in a Directive Prologue an ExpressionStatement that is not a Use Strict Directive or which does not have a meaning defined by the
implementation.

There are certain built-in objects available whenever an ECMAScript program begins execution. One, the global object, is part
of the lexical environment of the executing program. Others are accessible as initial properties of the
global object.

Unless specified otherwise, the [[Class]] internal property of a built-in object is "Function" if that built-in
object has a [[Call]] internal property, or "Object" if that built-in object does not have a [[Call]] internal
property. Unless specified otherwise, the [[Extensible]] internal property of a built-in object initially has the value
true.

Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are
functions intended for use with the new operator. For each built-in function, this specification describes the
arguments required by that function and properties of the Function object. For each built-in constructor, this specification
furthermore describes properties of the prototype object of that constructor and properties of specific object instances
returned by a new expression that invokes that constructor.

Unless otherwise specified in the description of a particular function, if a function or constructor described in this clause
is given fewer arguments than the function is specified to require, the function or constructor shall behave exactly as if it
had been given sufficient additional arguments, each such argument being the undefined value.

Unless otherwise specified in the description of a particular function, if a function or constructor described in this clause
is given more arguments than the function is specified to allow, the extra arguments are evaluated by the call and then ignored
by the function. However, an implementation may define implementation specific behaviour relating to such arguments as long as
the behaviour is not the throwing of a TypeError exception that is predicated simply on the presence of an extra
argument.

NOTE Implementations that add additional capabilities to the set of built-in functions are
encouraged to do so by adding new functions rather than adding new parameters to existing functions.

Every built-in function and every built-in constructor has the Function prototype object, which is the initial value of the
expression Function.prototype (15.3.4), as the value of its [[Prototype]] internal
property.

Unless otherwise specified every built-in prototype object has the Object prototype object, which is the initial value of the
expression Object.prototype (15.2.4), as the value of its [[Prototype]] internal
property, except the Object prototype object itself.

None of the built-in functions described in this clause that are not constructors shall implement the [[Construct]] internal
method unless otherwise specified in the description of a particular function. None of the built-in functions described in this
clause shall have a prototype property unless otherwise specified in the description of a particular function.

This clause generally describes distinct behaviours for when a constructor is “called as a function” and for when
it is “called as part of a new expression”. The “called as a function” behaviour corresponds to
the invocation of the constructor’s [[Call]] internal method and the “called as part of a new expression”
behaviour corresponds to the invocation of the constructor’s [[Construct]] internal method.

Every built-in Function object described in this clause—whether as a constructor, an ordinary function, or
both—has a length property whose value is an integer. Unless otherwise specified, this value is equal to the
largest number of named arguments shown in the subclause headings for the function description, including optional
parameters.

NOTE For example, the Function object that is the initial value of the slice property of the String prototype object is described under the subclause heading
“String.prototype.slice (start, end)” which shows the two named arguments start and end; therefore the value of
the length property of that Function object is 2.

In every case, the length property of a built-in Function object described in this clause has the attributes
{ [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }. Every other property
described in this clause has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
true } unless otherwise specified.

The global object does not have a [[Construct]] internal property; it is not possible to use the global object as a
constructor with the new operator.

The global object does not have a [[Call]] internal property; it is not possible to invoke the global object as a
function.

The values of the [[Prototype]] and [[Class]] internal properties of the global object are implementation-dependent.

In addition to the properties defined in this specification the global object may have additional host defined properties.
This may include a property whose value is the global object itself; for example, in the HTML document object model the
window property of the global object is the global object itself.

The parseInt function produces an integer value dictated by interpretation of the contents of the
string argument according to the specified radix. Leading white space in string is
ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the character pairs 0x or 0X, in which case
a radix of 16 is assumed. If radix is 16, the number may also
optionally begin with the character pairs 0x or 0X.

Let S be a newly created substring of inputString consisting of the first character that is not a
StrWhiteSpaceChar and all characters following that character. (In other words, remove leading white space.)
If inputString does not contain any such characters, let S be the empty string.

Let sign be 1.

If S is not empty and the first character of S is a minus sign -, let sign be
−1.

If S is not empty and the first character of S is a plus sign + or a minus sign -, then
remove the first character from S.

If the length of S is at least 2 and the first two characters of S are either
“0x” or “0X”, then remove the first two characters from S and let
R = 16.

If S contains any character that is not a radix-R digit, then let Z be the substring of
S consisting of all characters before the first such character; otherwise, let Z be S.

If Z is empty, return NaN.

Let mathInt be the mathematical integer value that is represented by Z in radix-R notation,
using the letters A-Z and a-z for digits with values 10 through 35. (However, if
R is 10 and Z contains more than 20 significant digits, every significant digit after the 20th may be
replaced by a 0 digit, at the option of the implementation; and if R is not 2, 4, 8, 10, 16, or 32,
then mathInt may be an implementation-dependent approximation to the mathematical integer value that is
represented by Z in radix-R notation.)

Let number be the Number value for mathInt.

Return sign × number.

NOTEparseInt may interpret only a leading portion of string as an
integer value; it ignores any characters that cannot be interpreted as part of the notation of an integer, and no
indication is given that any such characters were ignored.

Let trimmedString be a substring of inputString consisting of the leftmost character that is not a
StrWhiteSpaceChar and all characters to the right of that character. (In other words, remove leading white
space.) If inputString does not contain any such characters, let trimmedString be the empty
string.

If neither trimmedString nor any prefix of trimmedString satisfies the syntax of a
StrDecimalLiteral (see 9.3.1), return NaN.

Let numberString be the longest prefix of trimmedString, which might be trimmedString itself,
that satisfies the syntax of a StrDecimalLiteral.

Return the Number value for the MV of numberString.

NOTEparseFloat may interpret only a leading portion of string as a
Number value; it ignores any characters that cannot be interpreted as part of the notation of an decimal literal, and no
indication is given that any such characters were ignored.

Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and transport
protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any
support for using URIs except for functions that encode and decode URIs as described in 15.1.3.1, 15.1.3.2, 15.1.3.3 and 15.1.3.4.

NOTE Many implementations of ECMAScript provide additional functions and methods that
manipulate web pages; these functions are beyond the scope of this standard.

A URI is composed of a sequence of components separated by component separators. The general form is:

Scheme:First/Second;Third?Fourth

where the italicised names represent components and “:”, “/”,
“;” and “?” are reserved characters used as separators. The
encodeURI and decodeURI functions are intended to work with complete URIs; they assume that any
reserved characters in the URI are intended to have special meaning and so are not encoded. The
encodeURIComponent and decodeURIComponent functions are intended to work with the individual
component parts of a URI; they assume that any reserved characters represent text and so must be encoded so that they are
not interpreted as reserved characters when the component is part of a complete URI.

The following lexical grammar specifies the form of encoded URIs.

Syntax

uri:::

uriCharactersopt

uriCharacters:::

uriCharacteruriCharactersopt

uriCharacter:::

uriReserved

uriUnescaped

uriEscaped

uriReserved:::one of

;/?:@&=+$,

uriUnescaped:::

uriAlpha

DecimalDigit

uriMark

uriEscaped:::

%HexDigitHexDigit

uriAlpha:::one of

abcdefghijklmnopqrstuvwxyz

ABCDEFGHIJKLMNOPQRSTUVWXYZ

uriMark:::one of

-_.!~*'()

NOTE The above syntax is based upon RFC 2396 and does not reflect changes introduced by the
more recent RFC 3986.

When a character to be included in a URI is not listed above or is not intended to have the special meaning sometimes
given to the reserved characters, that character must be encoded. The character is transformed into its UTF-8 encoding, with
surrogate pairs first converted from UTF-16 to the corresponding code point value. (Note that for code units in the range
[0,127] this results in a single octet with the same value.) The resulting sequence of octets is then transformed into a
String with each octet represented by an escape sequence of the form “%xx”.

The encoding and escaping process is described by the abstract operation Encode taking two String arguments
string and unescapedSet.

Let strLen be the number of characters in string.

Let R be the empty String.

Let k be 0.

Repeat

If k equals strLen, return R.

Let C be the character at position k within string.

If C is in unescapedSet, then

Let S be a String containing only the character C.

Let R be a new String value computed by concatenating the previous value of R and S.

Else, C is not in unescapedSet

If the code unit value of C is not less than 0xDC00 and not greater than 0xDFFF, throw a
URIError exception.

If the code unit value of C is less than 0xD800 or greater than 0xDBFF, then

Let V be the code unit value of C.

Else,

Increase k by 1.

If k equals strLen, throw a URIError exception.

Let kChar be the code unit value of the character at position k within string.

If kChar is less than 0xDC00 or greater than 0xDFFF, throw a URIError exception.

Let Octets be the array of octets resulting by applying the UTF-8 transformation to V, and let
L be the array size.

Let j be 0.

Repeat, while j < L

Let jOctet be the value at position j within Octets.

Let S be a String containing three characters “%XY” where XY are
two uppercase hexadecimal digits encoding the value of jOctet.

Let R be a new String value computed by concatenating the previous value of R and
S.

Increase j by 1.

Increase k by 1.

The unescaping and decoding process is described by the abstract operation Decode taking two String arguments
string and reservedSet.

Let strLen be the number of characters in string.

Let R be the empty String.

Let k be 0.

Repeat

If k equals strLen, return R.

Let C be the character at position k within string.

If C is not ‘%’, then

Let S be the String containing only the character C.

Else, C is ‘%’

Let start be k.

If k + 2 is greater than or equal to strLen, throw a URIError exception.

If the characters at position (k+1) and (k + 2) within string do not represent
hexadecimal digits, throw a URIError exception.

Let B be the 8-bit value represented by the two hexadecimal digits at position (k + 1) and
(k + 2).

Increment k by 2.

If the most significant bit in B is 0, then

Let C be the character with code unit value B.

If C is not in reservedSet, then

Let S be the String containing only the character C.

Else, C is in reservedSet

Let S be the substring of string from position start to position k
included.

Else, the most significant bit in B is 1

Let n be the smallest non-negative number such that (B << n) & 0x80 is
equal to 0.

If n equals 1 or n is greater than 4, throw a URIError exception.

Let Octets be an array of 8-bit integers of size n.

Put B into Octets at position 0.

If k + (3 × (n – 1)) is greater than or equal to strLen, throw a
URIError exception.

Let j be 1.

Repeat, while j < n

Increment k by 1.

If the character at position k is not ‘%’, throw a URIError exception.

If the characters at position (k +1) and (k + 2) within string do not represent
hexadecimal digits, throw a URIError exception.

Let B be the 8-bit value represented by the two hexadecimal digits at position (k + 1)
and (k + 2).

If the two most significant bits in B are not 10, throw a URIError exception.

Increment k by 2.

Put B into Octets at position j.

Increment j by 1.

Let V be the value obtained by applying the UTF-8 transformation to Octets, that is, from an
array of octets into a 21-bit value. If Octets does not contain a valid UTF-8 encoding of a Unicode
code point throw an URIError exception.

If V is less than 0x10000, then

Let C be the character with code unit value V.

If C is not in reservedSet, then

Let S be the String containing only the character C.

Else, C is in reservedSet

Let S be the substring of string from position start to position k
included.

Else, V is ≥ 0x10000

Let L be (((V – 0x10000) & 0x3FF) + 0xDC00).

Let H be ((((V – 0x10000) >> 10) & 0x3FF) + 0xD800).

Let S be the String containing the two characters with code unit values H and
L.

Let R be a new String value computed by concatenating the previous value of R and S.

Increase k by 1.

NOTE This syntax of Uniform Resource Identifiers is based upon RFC 2396 and does not reflect
the more recent RFC 3986 which replaces RFC 2396. A formal description and implementation of UTF-8 is given in RFC
3629.

In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a "sequence" of one has the
higher-order bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n>1,
the initial octet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bits of that octet contain
bits from the value of the character to be encoded. The following octets all have the higher-order bit set to 1 and the
following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The possible UTF-8
encodings of ECMAScript characters are specified in Table 21.

Table 21 — UTF-8 Encodings

Code Unit Value

Representation

1st Octet

2nd Octet

3rd Octet

4th Octet

0x0000 - 0x007F

000000000zzzzzzz

0zzzzzzz

0x0080 - 0x07FF

00000yyy yyzzzzzz

110yyyyy

10zzzzzz

0x0800 - 0xD7FF

xxxxyyyy yyzzzzzz

1110xxxx

10yyyyyy

10zzzzzz

0xD800 - 0xDBFF

followed by

0xDC00 – 0xDFFF

110110vv vvwwwwxx

followed by

110111yy yyzzzzzz

11110uuu

10uuwwww

10xxyyyy

10zzzzzz

0xD800 - 0xDBFF

not followed by

0xDC00 – 0xDFFF

causes URIError

0xDC00 – 0xDFFF

causes URIError

0xE000 - 0xFFFF

xxxxyyyy yyzzzzzz

1110xxxx

10yyyyyy

10zzzzzz

Where

uuuuu = vvvv + 1

to account for the addition of 0x10000 as in Surrogates, section 3.7, of the Unicode Standard.

The range of code unit values 0xD800-0xDFFF is used to encode surrogate pairs; the above transformation combines a UTF-16
surrogate pair into a UTF-32 representation and encodes the resulting 21-bit value in UTF-8. Decoding reconstructs the
surrogate pair.

RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence C0 80 must not decode
into the character U+0000. Implementations of the Decode algorithm are required to throw a URIError when encountering
such invalid sequences.

The decodeURI function computes a new version of a URI in which each escape sequence and UTF-8 encoding of
the sort that might be introduced by the encodeURI function is replaced with the character that it
represents. Escape sequences that could not have been introduced by encodeURI are not replaced.

When the decodeURI function is called with one argument encodedURI, the following steps are
taken:

The decodeURIComponent function computes a new version of a URI in which each escape sequence and UTF-8
encoding of the sort that might be introduced by the encodeURIComponent function is replaced with the
character that it represents.

When the decodeURIComponent function is called with one argument encodedURIComponent, the
following steps are taken:

The encodeURI function computes a new version of a URI in which each instance of certain characters is
replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the character.

When the encodeURI function is called with one argument uri, the following steps
are taken:

The encodeURIComponent function computes a new version of a URI in which each instance of certain
characters is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the character.

When the encodeURIComponent function is called with one argument uriComponent, the
following steps are taken:

NOTE If O is a String instance, the set of own properties processed in step 4
includes the implicit properties defined in 15.5.5.2 that correspond to character positions
within the object’s [[PrimitiveValue]] String.

The defineProperty function is used to add an own property and/or update the attributes of an existing own
property of an object. When the defineProperty function is called, the following steps are taken:

The defineProperties function is used to add own properties and/or update the attributes of existing own
properties of an object. When the defineProperties function is called, the following steps are taken:

The value of the [[Prototype]] internal property of the Object prototype object is null, the value of the
[[Class]] internal property is "Object", and the initial value of the [[Extensible]] internal property is
true.

Return the result of calling the [[Call]] internal method of toString passing O as the this
value and no arguments.

NOTE 1 This function is provided to give all Objects a generic toLocaleString
interface, even though not all may use it. Currently, Array, Number, and Date
provide their own locale-sensitive toLocaleString methods.

NOTE 2 The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.

Let O be the result of calling ToObject passing the this value as the
argument.

Let desc be the result of calling the [[GetOwnProperty]] internal method of O passing P as the
argument.

If desc is undefined, return false.

Return true.

NOTE 1 Unlike [[HasProperty]] (8.12.6), this method does not
consider objects in the prototype chain.

NOTE 2 The ordering of steps 1 and 2 is chosen to ensure that any exception that would have
been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this
value is undefined or null.

When the isPrototypeOf method is called with argument V, the following steps are taken:

If V is not an object, return false.

Let O be the result of calling ToObject passing the this value as the
argument.

Repeat

Let V be the value of the [[Prototype]] internal property of V.

if V is null, return false

If O and V refer to the same object, return true.

NOTE The ordering of steps 1 and 2 is chosen to preserve the behaviour specified by previous
editions of this specification for the case where V is not an object and the this value is undefined or null.

Let O be the result of calling ToObject passing the this value as the
argument.

Let desc be the result of calling the [[GetOwnProperty]] internal method of O passing P as the
argument.

If desc is undefined, return false.

Return the value of desc.[[Enumerable]].

NOTE 1 This method does not consider objects in the prototype chain.

NOTE 2 The ordering of steps 1 and 2 is chosen to ensure that any exception that would have
been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this
value is undefined or null.

When Function is called as a function rather than as a constructor, it creates and initialises a new
Function object. Thus the function call Function(…) is
equivalent to the object creation expression new
Function(…) with the same arguments.

When the Function function is called with some arguments p1, p2, … ,
pn, body (where n might be 0, that is,
there are no “p” arguments, and where body might also not be provided), the following
steps are taken:

Create and return a new Function object as if the standard built-in constructor Function was used in a new
expression with the same arguments (15.3.2.1).

The last argument specifies the body (executable code) of a function; any preceding arguments specify formal
parameters.

When the Function constructor is called with some arguments p1, p2, … ,
pn, body (where n might be 0, that is,
there are no “p” arguments, and where body might also not be provided), the following
steps are taken:

Let argCount be the total number of arguments passed to this function invocation.

Return a new Function object created as specified in 13.2 passing P as the
FormalParameterListopt and body as the FunctionBody. Pass in the Global Environment as the Scope parameter and strict as the Strict
flag.

A prototype property is automatically created for every function, to provide for the possibility that the
function will be used as a constructor.

NOTE It is permissible but not necessary to have one argument for each formal parameter to be
specified. For example, all three of the following expressions produce the same result:

The Function constructor is itself a Function object and its [[Class]] is "Function". The value of the
[[Prototype]] internal property of the Function constructor is the standard built-in Function prototype object (15.3.4).

The value of the [[Extensible]] internal property of the Function constructor is true.

The Function prototype object is itself a Function object (its [[Class]] is "Function") that, when invoked,
accepts any arguments and returns undefined.

The value of the [[Prototype]] internal property of the Function prototype object is the standard built-in Object
prototype object (15.2.4). The initial value of the [[Extensible]] internal property of the
Function prototype object is true.

The Function prototype object does not have a valueOf property of its own; however, it inherits the
valueOf property from the Object prototype Object.

An implementation-dependent representation of the function is returned. This representation has the syntax of a FunctionDeclaration. Note in particular that the use and placement of white space, line terminators, and
semicolons within the representation String is implementation-dependent.

The toString function is not generic; it throws a TypeError exception if its this value is
not a Function object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

Let nextArg be the result of calling the [[Get]] internal method of argArray with indexName
as the argument.

Append nextArg as the last element of argList.

Set index to index + 1.

Return the result of calling the [[Call]] internal method of func, providing thisArg as the
this value and argList as the list of arguments.

The length property of the apply method is 2.

NOTE The thisArg value is passed without modification as the this value. This is a
change from Edition 3, where a undefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value.

If this method was called with more than one argument then in left to right order starting with arg1 append
each argument as the last element of argList

Return the result of calling the [[Call]] internal method of func, providing thisArg as the
this value and argList as the list of arguments.

The length property of the call method is 1.

NOTE The thisArg value is passed without modification as the this value. This is a
change from Edition 3, where a undefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value.

In addition to the required internal properties, every function instance has a [[Call]] internal property and in most
cases uses a different version of the [[Get]] internal property. Depending on how they are created (see 8.6.2, 13.2, 15, and 15.3.4.5), function instances
may have a [[HasInstance]] internal property, a [[Scope]] internal property, a [[Construct]] internal property, a
[[FormalParameters]] internal property, a [[Code]] internal property, a [[TargetFunction]] internal property, a
[[BoundThis]] internal property, and a [[BoundArgs]] internal property.

The value of the [[Class]] internal property is "Function".

Function instances that correspond to strict mode functions (13.2) and function instances created
using the Function.prototype.bind method (15.3.4.5) have properties named
“caller” and “arguments” that throw a TypeError exception. An ECMAScript implementation must
not associate any implementation specific behaviour with accesses of these properties from strict mode function code.

The value of the length property is an integer that indicates the “typical” number of
arguments expected by the function. However, the language permits the function to be invoked with some other number of
arguments. The behaviour of a function when invoked on a number of arguments other than the number specified by its
length property depends on the function. This property has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: false }.

The value of the prototype property is used to initialise the [[Prototype]] internal property of a newly
created object before the Function object is invoked as a constructor for that newly created object. This property has the
attribute { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

NOTE Function objects created using Function.prototype.bind do not have a
prototype property.

Array objects give special treatment to a certain class of property names. A property name P (in the form of a
String value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 232−1. A property whose property name is an array index is also
called an element. Every Array object has a length property whose value is always a nonnegative integer
less than 232. The value of the length property is
numerically greater than the name of every property whose name is an array index; whenever a property of an Array object is
created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever a property
is added whose name is an array index, the length property is changed, if necessary, to be one more than the
numeric value of that array index; and whenever the length property is changed, every property whose name is an
array index whose value is not smaller than the new length is automatically deleted. This constraint applies only to own
properties of an Array object and is unaffected by length or array index properties that may be inherited from
its prototypes.

An object, O, is said to be sparse if the following algorithm returns true:

Let len be the result of calling the [[Get]] internal method of O with argument "length".

When Array is called as a function rather than as a constructor, it creates and initialises a new Array
object. Thus the function call Array(…) is equivalent to
the object creation expression new Array(…) with the
same arguments.

This description applies if and only if the Array constructor is given no arguments or at least two arguments.

The [[Prototype]] internal property of the newly constructed object is set to the original Array prototype object, the
one that is the initial value of Array.prototype (15.4.3.1).

The [[Class]] internal property of the newly constructed object is set to "Array".

The [[Extensible]] internal property of the newly constructed object is set to true.

The length property of the newly constructed object is set to the number of arguments.

The 0 property of the newly constructed object is set to item0 (if supplied); the
1 property of the newly constructed object is set to item1 (if supplied); and, in general, for as
many arguments as there are, the k property of the newly constructed object is set to argument k,
where the first argument is considered to be argument number 0. These properties all have the attributes
{[[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.

The [[Prototype]] internal property of the newly constructed object is set to the original Array prototype object, the
one that is the initial value of Array.prototype (15.4.3.1). The [[Class]]
internal property of the newly constructed object is set to "Array". The [[Extensible]] internal property of
the newly constructed object is set to true.

If the argument len is a Number and ToUint32(len) is equal to len, then the length property of the
newly constructed object is set to ToUint32(len). If the argument len is a Number and ToUint32(len) is not equal to len, a RangeError
exception is thrown.

If the argument len is not a Number, then the length property of the newly constructed object
is set to 1 and the 0 property of the newly constructed object is set to len with
attributes {[[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.

The isArray function takes one argument arg, and returns the Boolean value true if the argument is an
object whose class internal property is "Array"; otherwise it returns false. The following steps are
taken:

The value of the [[Prototype]] internal property of the Array prototype object is the standard built-in Object prototype
object (15.2.4).

The Array prototype object is itself an array; its [[Class]] is "Array", and it has a length
property (whose initial value is +0) and the special [[DefineOwnProperty]] internal method described in 15.4.5.1.

In following descriptions of functions that are properties of the Array prototype object, the phrase “this
object” refers to the object that is the this value for the invocation of the function. It is permitted for the
this to be an object for which the value of the [[Class]] internal property is not "Array".

NOTE The Array prototype object does not have a valueOf property of its own;
however, it inherits the valueOf property from the standard built-in Object prototype Object.

Let func be the result of calling the [[Get]] internal method of array with argument
"join".

If IsCallable(func) is false, then let func be the standard built-in
method Object.prototype.toString (15.2.4.2).

Return the result of calling the [[Call]] internal method of func providing array as the this
value and an empty arguments list.

NOTE The toString function is intentionally generic; it does not require that
its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the toString function can be applied successfully to a host object is implementation-dependent.

The elements of the array are converted to Strings using their toLocaleString methods, and these Strings
are then concatenated, separated by occurrences of a separator String that has been derived in an implementation-defined
locale-specific way. The result of calling this function is intended to be analogous to the result of
toString, except that the result of this function is intended to be locale-specific.

The result is calculated as follows:

Let array be the result of calling ToObject passing the this value as the
argument.

Let arrayLen be the result of calling the [[Get]] internal method of array with argument
"length".

Let R be the result of calling the [[Call]] internal method of func providing
elementObj as the this value and an empty arguments list.

Let R be a String value produced by concatenating S and R.

Increase k by 1.

Return R.

NOTE 1 The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.

NOTE 2 The toLocaleString function is intentionally generic; it does not require
that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a
method. Whether the toLocaleString function can be applied successfully to a host object is
implementation-dependent.

When the concat method is called with zero or more arguments item1, item2, etc., it
returns an array containing the array elements of the object followed by the array elements of each argument in order.

The following steps are taken:

Let O be the result of calling ToObject passing the this value as the
argument.

Let A be a new array created as if by the expression new Array() where Array is the
standard built-in constructor with that name.

Let n be 0.

Let items be an internal List whose first element is O and whose subsequent
elements are, in left to right order, the arguments that were passed to this function invocation.

Repeat, while items is not empty

Remove the first element from items and let E be the value of the element.

If the value of the [[Class]] internal property of E is "Array", then

Let k be 0.

Let len be the result of calling the [[Get]] internal method of E with argument
"length".

NOTE The concat function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the concat function can be applied successfully to a host object is implementation-dependent.

The elements of the array are converted to Strings, and these Strings are then concatenated, separated by occurrences
of the separator. If no separator is provided, a single comma is used as the separator.

The join method takes one argument, separator, and performs the following steps:

Let O be the result of calling ToObject passing the this value as the
argument.

Let lenVal be the result of calling the [[Get]] internal method of O with argument
"length".

Let element0 be the result of calling the [[Get]] internal method of O with argument
"0".

If element0 is undefined or null, let R be the empty String; otherwise, Let R be
ToString(element0).

Let k be 1.

Repeat, while k < len

Let S be the String value produced by concatenating R and sep.

Let element be the result of calling the [[Get]] internal method of O with argument ToString(k).

If element is undefined or null, Let next be the empty String; otherwise, let
next be ToString(element).

Let R be a String value produced by concatenating S and next.

Increase k by 1.

Return R.

The length property of the join method is 1.

NOTE The join function is intentionally generic; it does not require that its
this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
Whether the join function can be applied successfully to a host object is implementation-dependent.

Let element be the result of calling the [[Get]] internal method of O with argument
indx.

Call the [[Delete]] internal method of O with arguments indx and true.

Call the [[Put]] internal method of O with arguments "length", indx, and true.

Return element.

NOTE The pop function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the pop function can be applied successfully to a host object is implementation-dependent.

Let items be an internal List whose elements are, in left to right order, the
arguments that were passed to this function invocation.

Repeat, while items is not empty

Remove the first element from items and let E be the value of the element.

Call the [[Put]] internal method of O with arguments ToString(n), E,
and true.

Increase n by 1.

Call the [[Put]] internal method of O with arguments "length", n, and true.

Return n.

The length property of the push method is 1.

NOTE The push function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the push function can be applied successfully to a host object is implementation-dependent.

NOTE The reverse function is intentionally generic; it does not require that its
this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
Whether the reverse function can be applied successfully to a host object is implementation-dependent.

NOTE The shift function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the shift function can be applied successfully to a host object is implementation-dependent.

The slice method takes two arguments, start and end, and returns an array containing
the elements of the array from element start up to, but not including, element end (or through the
end of the array if end is undefined). If start is negative, it is treated as length+start where length is the length of the array.
If end is negative, it is treated as length+end
where length is the length of the array. The following steps are taken:

Let O be the result of calling ToObject passing the this value as the
argument.

Let A be a new array created as if by the expression new Array() where Array is the
standard built-in constructor with that name.

Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".

NOTE The slice function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the slice function can be applied successfully to a host object is implementation-dependent.

The elements of this array are sorted. The sort is not necessarily stable (that is, elements that compare equal do not
necessarily remain in their original order). If comparefn is not undefined, it should be a function that
accepts two arguments x and y and returns a negative value if x<y, zero if x=y, or a positive value if x>y.

Let obj be the result of calling ToObject passing the this value as the
argument.

Let len be the result of applying Uint32 to the result of calling the [[Get]] internal method of
obj with argument "length".

If comparefn is not undefined and is not a consistent comparison function for the elements of this
array (see below), the behaviour of sort is implementation-defined.

Let proto be the value of the [[Prototype]] internal property of obj. If proto is not
null and there exists an integer j such that all of the conditions below are satisfied then the
behaviour of sort is implementation-defined:

The result of calling the [[HasProperty]] internal method of proto with argument ToString(j) is true.

The behaviour of sort is also implementation defined if obj is sparse and any of the following
conditions are true:

The [[Extensible]] internal property of obj is false.

Any array index property of obj whose name is a nonnegative integer less than len is a data
property whose [[Configurable]] attribute is false.

The behaviour of sort is also implementation defined if any array index property of obj whose
name is a nonnegative integer less than len is an accessor property or is a data property whose [[Writable]]
attribute is false.

Otherwise, the following steps are taken.

Perform an implementation-dependent sequence of calls to the [[Get]] , [[Put]], and [[Delete]] internal methods of
obj and to SortCompare (described below), where the first argument for each call to [[Get]], [[Put]], or
[[Delete]] is a nonnegative integer less than len and where the arguments for calls to SortCompare are
results of previous calls to the [[Get]] internal method. The throw argument to the [[Put]] and [[Delete]] internal
methods will be the value true. If obj is not sparse then [[Delete]] must not be called.

Return obj.

The returned object must have the following two properties.

There must be some mathematical permutation π of the
nonnegative integers less than len, such that for every nonnegative integer j less than
len, if property old[j] existed, then new[π(j)] is exactly the same value as old[j],. But if property old[j] did not exist, then new[π(j)]
does not exist.

Then for all nonnegative integers j and k, each less than len, if SortCompare(j,k) < 0 (see SortCompare below), then π(j) <π(k).

Here the notation old[j] is used to refer to the hypothetical
result of calling the [[Get]] internal method of obj with argument j before this function is
executed, and the notation new[j] to refer to the hypothetical
result of calling the [[Get]] internal method of obj with argument j after this function has been
executed.

A function comparefn is a consistent comparison function for a set of values S if all of the
requirements below are met for all values a, b, and c (possibly the same value) in the
set S: The notation a <CFb means comparefn(a,b) < 0; a =CFb means comparefn(a,b) = 0 (of either sign); and a >CFb means comparefn(a,b) > 0.

Calling comparefn(a,b) always returns the same value v when given a specific pair of
values a and b as its two arguments. Furthermore, Type(v) is Number, and
v is not NaN. Note that this implies that exactly one of a <CFb,
a =CFb, and a >CFb will be true for a given
pair of a and b.

Calling comparefn(a,b) does not modify the this object.

a =CFa (reflexivity)

If a =CFb, then b =CFa (symmetry)

If a =CFb and b =CFc, then
a =CFc (transitivity of =CF)

If a <CFb and b <CFc, then
a <CFc (transitivity of <CF)

If a >CFb and b >CFc, then
a >CFc (transitivity of >CF)

NOTE The above conditions are necessary and sufficient to ensure that comparefn
divides the set S into equivalence classes and that these equivalence classes are totally ordered.

When the SortCompare abstract operation is called with two arguments j and k, the following steps
are taken:

NOTE 1 Because non-existent property values always compare greater than undefined
property values, and undefined always compares greater than any other value, undefined property values always
sort to the end of the result, followed by non-existent property values.

NOTE 2 The sort function is intentionally generic; it does not require that its
this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
Whether the sort function can be applied successfully to a host object is implementation-dependent.

When the splice method is called with two or more arguments start, deleteCount and
(optionally) item1, item2, etc., the deleteCount elements of the array starting at array
index start are replaced by the arguments item1, item2, etc. An Array object containing
the deleted elements (if any) is returned. The following steps are taken:

Let O be the result of calling ToObject passing the this value as the
argument.

Let A be a new array created as if by the expression new Array()where Array is the
standard built-in constructor with that name.

Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".

NOTE The splice function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the splice function can be applied successfully to a host object is implementation-dependent.

Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument
from.

If fromPresent is true, then

Let fromValue be the result of calling the [[Get]] internal method of O with argument
from.

Call the [[Put]] internal method of O with arguments to, fromValue, and
true.

Else, fromPresent is false

Call the [[Delete]] internal method of O with arguments to, and true.

Decrease k by 1.

Let j be 0.

Let items be an internal List whose elements are, in left to right order, the
arguments that were passed to this function invocation.

Repeat, while items is not empty

Remove the first element from items and let E be the value of that element.

Call the [[Put]] internal method of O with arguments ToString(j), E,
and true.

Increase j by 1.

Call the [[Put]] internal method of O with arguments "length", len+argCount, and true.

Return len+argCount.

The length property of the unshift method is 1.

NOTE The unshift function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the unshift function can be applied successfully to a host object is implementation-dependent.

indexOf compares searchElement to the elements of the array, in ascending order, using the
internal Strict Equality Comparison Algorithm (11.9.6), and if found at one or more positions,
returns the index of the first such position; otherwise, -1 is returned.

The optional second argument fromIndex defaults to 0 (i.e. the whole array is searched). If it is greater
than or equal to the length of the array, -1 is returned, i.e. the array will not be searched. If it is negative, it is
used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0, the
whole array will be searched.

When the indexOf method is called with one or two arguments, the following steps are taken:

Let O be the result of calling ToObject passing the this value as the
argument.

Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".

Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument ToString(k).

If kPresent is true, then

Let elementK be the result of calling the [[Get]] internal method of O with the argument ToString(k).

Let same be the result of applying the Strict Equality Comparison Algorithm to searchElement
and elementK.

If same is true, return k.

Increase k by 1.

Return -1.

The length property of the indexOf method is 1.

NOTE The indexOf function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the indexOf function can be applied successfully to a host object is implementation-dependent.

lastIndexOf compares searchElement to the elements of the array in descending order using the
internal Strict Equality Comparison Algorithm (11.9.6), and if found at one or more positions,
returns the index of the last such position; otherwise, -1 is returned.

The optional second argument fromIndex defaults to the array's length minus one (i.e. the whole array is
searched). If it is greater than or equal to the length of the array, the whole array will be searched. If it is negative,
it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0,
-1 is returned.

When the lastIndexOf method is called with one or two arguments, the following steps are taken:

Let O be the result of calling ToObject passing the this value as the
argument.

Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".

Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument ToString(k).

If kPresent is true, then

Let elementK be the result of calling the [[Get]] internal method of O with the argument ToString(k).

Let same be the result of applying the Strict Equality Comparison Algorithm to searchElement
and elementK.

If same is true, return k.

Decrease k by 1.

Return -1.

The length property of the lastIndexOf method is 1.

NOTE The lastIndexOf function is intentionally generic; it does not require that
its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the lastIndexOf function can be applied successfully to a host object is
implementation-dependent.

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the
Boolean value true or false. every calls callbackfn once for each element present in
the array, in ascending order, until it finds one where callbackfn returns false. If such an element is
found, every immediately returns false. Otherwise, if callbackfn returned true for
all elements, every will return true. callbackfn is called only for elements of the array
which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of
callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the
object being traversed.

every does not directly mutate the object on which it is called but the object may be mutated by the calls
to callbackfn.

The range of elements processed by every is set before the first call to callbackfn. Elements
which are appended to the array after the call to every begins will not be visited by callbackfn.
If existing elements of the array are changed, their value as passed to callbackfn will be the value at the
time every visits them; elements that are deleted after the call to every begins and before
being visited are not visited. every acts like the "for all" quantifier in mathematics. In particular, for an
empty array, it returns true.

When the every method is called with one or two arguments, the following steps are taken:

Let O be the result of calling ToObject passing the this value as the
argument.

Let lenValue be the result of calling the [[Get]] internal method of O with the argument
"length".

NOTE The every function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the every function can be applied successfully to a host object is implementation-dependent.

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the
Boolean value true or false. some calls callbackfn once for each element present in
the array, in ascending order, until it finds one where callbackfn returns true. If such an element is
found, some immediately returns true. Otherwise, some returns false.
callbackfn is called only for elements of the array which actually exist; it is not called for missing elements
of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of
callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the
object being traversed.

some does not directly mutate the object on which it is called but the object may be mutated by the calls
to callbackfn.

The range of elements processed by some is set before the first call to callbackfn. Elements
that are appended to the array after the call to some begins will not be visited by callbackfn. If
existing elements of the array are changed, their value as passed to callbackfn will be the value at the time
that some visits them; elements that are deleted after the call to some begins and before being
visited are not visited. some acts like the "exists" quantifier in mathematics. In particular, for an empty
array, it returns false.

When the some method is called with one or two arguments, the following steps are taken:

Let O be the result of calling ToObject passing the this value as the
argument.

Let lenValue be the result of calling the [[Get]] internal method of O with the argument
"length".

NOTE The some function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the some function can be applied successfully to a host object is implementation-dependent.

callbackfn should be a function that accepts three arguments. forEach calls
callbackfn once for each element present in the array, in ascending order. callbackfn is called only
for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of
callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the
object being traversed.

forEach does not directly mutate the object on which it is called but the object may be mutated by the
calls to callbackfn.

The range of elements processed by forEach is set before the first call to callbackfn. Elements
which are appended to the array after the call to forEach begins will not be visited by
callbackfn. If existing elements of the array are changed, their value as passed to callback will be the value
at the time forEach visits them; elements that are deleted after the call to forEach begins and
before being visited are not visited.

When the forEach method is called with one or two arguments, the following steps are taken:

Let O be the result of calling ToObject passing the this value as the
argument.

Let lenValue be the result of calling the [[Get]] internal method of O with the argument
"length".

Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
Pk.

If kPresent is true, then

Let kValue be the result of calling the [[Get]] internal method of O with argument
Pk.

Call the [[Call]] internal method of callbackfn with T as the this value and argument
list containing kValue, k, and O.

Increase k by 1.

Return undefined.

The length property of the forEach method is 1.

NOTE The forEach function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the forEach function can be applied successfully to a host object is implementation-dependent.

callbackfn should be a function that accepts three arguments. map calls callbackfn
once for each element in the array, in ascending order, and constructs a new Array from the results. callbackfn
is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of
callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the
object being traversed.

map does not directly mutate the object on which it is called but the object may be mutated by the calls
to callbackfn.

The range of elements processed by map is set before the first call to callbackfn. Elements
which are appended to the array after the call to map begins will not be visited by callbackfn. If
existing elements of the array are changed, their value as passed to callbackfn will be the value at the time
map visits them; elements that are deleted after the call to map begins and before being visited
are not visited.

When the map method is called with one or two arguments, the following steps are taken:

Let O be the result of calling ToObject passing the this value as the
argument.

Let lenValue be the result of calling the [[Get]] internal method of O with the argument
"length".

NOTE The map function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the map function can be applied successfully to a host object is implementation-dependent.

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the
Boolean value true or false. filter calls callbackfn once for each element in the
array, in ascending order, and constructs a new array of all the values for which callbackfn returns
true. callbackfn is called only for elements of the array which actually exist; it is not called for
missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of
callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the
object being traversed.

filter does not directly mutate the object on which it is called but the object may be mutated by the
calls to callbackfn.

The range of elements processed by filter is set before the first call to callbackfn. Elements
which are appended to the array after the call to filter begins will not be visited by callbackfn.
If existing elements of the array are changed their value as passed to callbackfn will be the value at the time
filter visits them; elements that are deleted after the call to filter begins and before being
visited are not visited.

When the filter method is called with one or two arguments, the following steps are taken:

Let O be the result of calling ToObject passing the this value as the
argument.

Let lenValue be the result of calling the [[Get]] internal method of O with the argument
"length".

NOTE The filter function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the filter function can be applied successfully to a host object is implementation-dependent.

callbackfn should be a function that takes four arguments. reduce calls the callback, as a
function, once for each element present in the array, in ascending order.

callbackfn is called with four arguments: the previousValue (or value from the previous call to
callbackfn), the currentValue (value of the current element), the currentIndex, and the object
being traversed. The first time that callback is called, the previousValue and currentValue can be one of
two values. If an initialValue was provided in the call to reduce, then previousValue will
be equal to initialValue and currentValue will be equal to the first value in the array. If no
initialValue was provided, then previousValue will be equal to the first value in the array and
currentValue will be equal to the second. It is a TypeError if the array contains no elements and
initialValue is not provided.

reduce does not directly mutate the object on which it is called but the object may be mutated by the
calls to callbackfn.

The range of elements processed by reduce is set before the first call to callbackfn. Elements
that are appended to the array after the call to reduce begins will not be visited by callbackfn.
If existing elements of the array are changed, their value as passed to callbackfn will be the value at the
time reduce visits them; elements that are deleted after the call to reduce begins and before
being visited are not visited.

When the reduce method is called with one or two arguments, the following steps are taken:

Let O be the result of calling ToObject passing the this value as the
argument.

Let lenValue be the result of calling the [[Get]] internal method of O with the argument
"length".

Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
Pk.

If kPresent is true, then

Let kValue be the result of calling the [[Get]] internal method of O with argument
Pk.

Let accumulator be the result of calling the [[Call]] internal method of callbackfn with
undefined as the this value and argument list containing accumulator, kValue,
k, and O.

Increase k by 1.

Return accumulator.

The length property of the reduce method is 1.

NOTE The reduce function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the reduce function can be applied successfully to a host object is implementation-dependent.

callbackfn should be a function that takes four arguments. reduceRight calls the callback, as a
function, once for each element present in the array, in descending order.

callbackfn is called with four arguments: the previousValue (or value from the previous call to
callbackfn), the currentValue (value of the current element), the currentIndex, and the
object being traversed. The first time the function is called, the previousValue and currentValue
can be one of two values. If an initialValue was provided in the call to reduceRight, then
previousValue will be equal to initialValue and currentValue will be equal to the last
value in the array. If no initialValue was provided, then previousValue will be equal to the last
value in the array and currentValue will be equal to the second-to-last value. It is a TypeError if the
array contains no elements and initialValue is not provided.

reduceRight does not directly mutate the object on which it is called but the object may be mutated by the
calls to callbackfn.

The range of elements processed by reduceRight is set before the first call to callbackfn.
Elements that are appended to the array after the call to reduceRight begins will not be visited by
callbackfn. If existing elements of the array are changed by callbackfn, their value as passed to
callbackfn will be the value at the time reduceRight visits them; elements that are deleted after
the call to reduceRight begins and before being visited are not visited.

When the reduceRight method is called with one or two arguments, the following steps are taken:

Let O be the result of calling ToObject passing the this value as the
argument.

Let lenValue be the result of calling the [[Get]] internal method of O with the argument
"length".

Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
Pk.

If kPresent is true, then

Let kValue be the result of calling the [[Get]] internal method of O with argument
Pk.

Let accumulator be the result of calling the [[Call]] internal method of callbackfn with
undefined as the this value and argument list containing accumulator, kValue,
k, and O.

Decrease k by 1.

Return accumulator.

The length property of the reduceRight method is 1.

NOTE The reduceRight function is intentionally generic; it does not require that
its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the reduceRight function can be applied successfully to a host object is
implementation-dependent.

In the following algorithm, the term “Reject” means
“If Throw is true, then throw a TypeError exception,
otherwise return false.”

When the [[DefineOwnProperty]] internal method of A is called with property P, Property DescriptorDesc, and Boolean flag Throw, the
following steps are taken:

Let oldLenDesc be the result of calling the [[GetOwnProperty]] internal method of A passing "length" as the argument. The result will never be undefined or an
accessor descriptor because Array objects are created with a length data property that cannot be deleted or
reconfigured.

Let oldLen be oldLenDesc.[[Value]].

If P is "length", then

If the [[Value]] field of Desc is absent, then

Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", Desc, and Throw as arguments.

NOTE Attempting to set the length property of an Array object to a value that is numerically
less than or equal to the largest numeric property name of an existing array indexed non-deletable property of the array
will result in the length being set to a numeric value that is one greater than that largest numeric property name. See
15.4.5.1.

Returns a String value containing as many characters as the number of arguments. Each argument specifies one character
of the resulting String, with the first argument specifying the first character, and so on, from left to right. An
argument is converted to a character by applying the operation ToUint16 (9.7) and regarding the resulting 16-bit integer as the code unit value of a character. If no arguments
are supplied, the result is the empty String.

Returns this String value. (Note that, for a String object, the toString method happens to return the same
thing as the valueOf method.)

The toString function is not generic; it throws a TypeError exception if its this value is
not a String or a String object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

The valueOf function is not generic; it throws a TypeError exception if its this value is
not a String or String object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

Returns a String containing the character at position pos in the String resulting from converting this
object to a String. If there is no character at that position, the result is the empty String. The result is a String
value, not a String object.

If pos is a value of Number type that is an integer, then the result of
x.charAt(pos) is equal to the result of
x.substring(pos,pos+1).

When the charAt method is called with one argument pos, the following steps are taken:

Return a String of length 1, containing one character from S, namely the character at position
position, where the first (leftmost) character in S is considered to be at position 0, the next one at
position 1, and so on.

NOTE The charAt function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.

Returns a Number (a nonnegative integer less than 216)
representing the code unit value of the character at position pos in the String resulting from converting this
object to a String. If there is no character at that position, the result is NaN.

When the charCodeAt method is called with one argument pos, the following steps are taken:

Return a value of Number type, whose value is the code unit value of the character at position position in
the String S, where the first (leftmost) character in S is considered to be at position 0, the next
one at position 1, and so on.

NOTE The charCodeAt function is intentionally generic; it does not require that
its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a
method.

When the concat method is called with zero or more arguments string1, string2, etc.,
it returns a String consisting of the characters of this object (converted to a String) followed by the characters of each
of string1, string2, etc. (where each argument is converted to a String). The result is a String
value, not a String object. The following steps are taken:

If searchString appears as a substring of the result of converting this object to a String, at one or more
positions that are greater than or equal to position, then the index of the smallest such position is returned;
otherwise, ‑1 is returned. If position is undefined, 0 is assumed, so as to search
all of the String.

The indexOf method takes two arguments, searchString and position, and performs the
following steps:

Let pos be ToInteger(position). (If position is undefined, this
step produces the value 0).

Let len be the number of characters in S.

Let start be min(max(pos, 0), len).

Let searchLen be the number of characters in searchStr.

Return the smallest possible integer k not smaller than start such that k+ searchLen is
not greater than len, and for all nonnegative integers j less than searchLen, the character at
position k+j of S is the same as the character at position j of searchStr; but if
there is no such integer k, then return the value -1.

The length property of the indexOf method is 1.

NOTE The indexOf function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.

If searchString appears as a substring of the result of converting this object to a String at one or more
positions that are smaller than or equal to position, then the index of the greatest such position is returned;
otherwise, ‑1 is returned. If position is undefined, the length of the String value
is assumed, so as to search all of the String.

The lastIndexOf method takes two arguments, searchString and position, and performs
the following steps:

Let numPos be ToNumber(position). (If position is undefined, this
step produces the value NaN).

If numPos is NaN, let pos be +∞; otherwise, let pos be ToInteger(numPos).

Let len be the number of characters in S.

Let start min(max(pos, 0), len).

Let searchLen be the number of characters in searchStr.

Return the largest possible nonnegative integer k not larger than start such that k+
searchLen is not greater than len, and for all nonnegative integers j less than
searchLen, the character at position k+j of S is the same as the character at position
j of searchStr; but if there is no such integer k, then return the value -1.

The length property of the lastIndexOf method is 1.

NOTE The lastIndexOf function is intentionally generic; it does not require that
its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.

When the localeCompare method is called with one argument that, it returns a Number other than
NaN that represents the result of a locale-sensitive String comparison of the this value (converted to a String)
with that (converted to a String). The two Strings are S and That. The two
Strings are compared in an implementation-defined fashion. The result is intended to order String values in the sort order
specified by the system default locale, and will be negative, zero, or positive, depending on whether S comes
before That in the sort order, the Strings are equal, or S comes after That in the sort order, respectively.

Before perform the comparisons the following steps are performed to prepare the Strings:

The localeCompare method, if considered as a function of two arguments this and that, is
a consistent comparison function (as defined in 15.4.4.11) on the set of all Strings.

The actual return values are implementation-defined to permit implementers to encode additional information in the
value, but the function is required to define a total ordering on all Strings and to return 0 when comparing
Strings that are considered canonically equivalent by the Unicode standard.

If no language-sensitive comparison at all is available from the host environment, this function may perform a bitwise
comparison.

NOTE 1 The localeCompare method itself is not directly suitable as an argument
to Array.prototype.sort because the latter requires a function of two arguments.

NOTE 2 This function is intended to rely on whatever language-sensitive comparison
functionality is available to the ECMAScript environment from the host environment, and to compare according to the
rules of the host environment’s current locale. It is strongly recommended that this function treat Strings that
are canonically equivalent according to the Unicode standard as identical (in other words, compare the Strings as if
they had both been converted to Normalised Form C or D first). It is also recommended that this function not honour
Unicode compatibility equivalences or decompositions.

NOTE 3 The second parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.

NOTE 4 The localeCompare function is intentionally generic; it does not require
that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.

Let string be the result of calling ToString, giving it the this value as its
argument.

If searchValue is a regular expression (an object whose [[Class]] internal property is
"RegExp"), do the following: If searchValue.global is false, then search string
for the first match of the regular expression searchValue. If searchValue.global is true,
then search string for all matches of the regular expression searchValue. Do the search in the same
manner as in String.prototype.match, including the update of searchValue.lastIndex.
Let m be the number of left capturing parentheses in searchValue (using NcapturingParens as specified in 15.10.2.1).

If searchValue is not a regular expression, let searchString be ToString(searchValue) and search string for the first
occurrence of searchString. Let m be 0.

If replaceValue is a function, then for each matched substring, call the function with the following
m + 3 arguments. Argument 1 is the substring that matched. If searchValue is a regular expression,
the next m arguments are all of the captures in the MatchResult (see 15.10.2.1).
Argument m + 2 is the offset within string where the match occurred, and argument m + 3
is string. The result is a String value derived from the original input by replacing each matched substring
with the corresponding return value of the function call, converted to a String if need be.

Otherwise, let newstring denote the result of converting replaceValue to a String. The result is
a String value derived from the original input String by replacing each matched substring with a String derived from
newstring by replacing characters in newstring by replacement text as specified in Table 22. These
$ replacements are done left-to-right, and, once such a replacement is performed, the new replacement text is
not subject to further replacements. For example, "$1,$2".replace(/(\$(\d))/g, "$$1-$1$2") returns
"$1-$11,$1-$22". A $ in newstring that does not match any of the forms below is left
as is.

Table 22 — Replacement Text Symbol Substitutions

Characters

Replacement text

$$

$

$&

The matched substring.

$‘

The portion of string that precedes the matched substring.

$’

The portion of string that follows the matched substring.

$n

The nth capture, where n is a single digit in the range 1 to 9 and $n is not followed by a decimal digit. If n≤m and the nth capture is undefined, use the empty String instead. If n>m, the result is implementation-defined.

$nn

The nnth capture, where nn is a two-digit decimal number in the range 01 to 99. If nn≤m and the nnth capture is undefined, use the empty String instead. If nn>m, the result is implementation-defined.

NOTE The replace function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.

Let string be the result of calling ToString, giving it the this value as its
argument.

If Type(regexp) is Object and the value of the [[Class]] internal property of
regexp is "RegExp", then let rx be regexp;

Else, let rx be a new RegExp object created as if by the expression new
RegExp(regexp) where RegExp is the standard built-in constructor with that
name.

Search the value string from its beginning for an occurrence of the regular expression pattern rx. Let
result be a Number indicating the offset within string where the pattern matched, or –1 if there
was no match. The lastIndex and global properties of regexp are ignored when
performing the search. The lastIndex property of regexp is left unchanged.

Return result.

NOTE The search function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.

The slice method takes two arguments, start and end, and returns a substring of the
result of converting this object to a String, starting from character position start and running to, but not
including, character position end (or through the end of the String if end is undefined). If
start is negative, it is treated as sourceLength+start where sourceLength is the length of the String. If
end is negative, it is treated as sourceLength+end where sourceLength is the length of the String. The result is a
String value, not a String object. The following steps are taken:

Returns an Array object into which substrings of the result of converting this object to a String have been stored. The
substrings are determined by searching from left to right for occurrences of separator; these occurrences are
not part of any substring in the returned array, but serve to divide up the String value. The value of
separator may be a String of any length or it may be a RegExp object (i.e., an object whose [[Class]] internal
property is "RegExp"; see 15.10).

The value of separator may be an empty String, an empty regular expression, or a regular expression that can
match an empty String. In this case, separator does not match the empty substring at the beginning or end of
the input String, nor does it match the empty substring at the end of the previous separator match. (For example, if
separator is the empty String, the String is split up into individual characters; the length of the result
array equals the length of the String, and each substring contains one character.) If separator is a regular
expression, only the first match at a given position of the this String is considered, even if backtracking could
yield a non-empty-substring match at that position. (For example, "ab".split(/a*?/) evaluates to the array
["a","b"], while "ab".split(/a*/) evaluates to the array["","b"].)

If the this object is (or converts to) the empty String, the result depends on whether separator can
match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one
element, which is the empty String.

If separator is a regular expression that contains capturing parentheses, then each time
separator is matched the results (including any undefined results) of the capturing parentheses are
spliced into the output array. For example,

If separator is undefined, then the result array contains just one String, which is the this
value (converted to a String). If limit is not undefined, then the output array is truncated so that it
contains no more than limit elements.

The substring method takes two arguments, start and end, and returns a substring of the
result of converting this object to a String, starting from character position start and running to, but not
including, character position end of the String (or through the end of the String is end is
undefined). The result is a String value, not a String object.

If either argument is NaN or negative, it is replaced with zero; if either argument is larger than the length of
the String, it is replaced with the length of the String.

Let S be the result of calling ToString, giving it the this value as its
argument.

Let L be a String where each character of L is either the Unicode lowercase equivalent of the
corresponding character of S or the actual corresponding character of S if no Unicode lowercase
equivalent exists.

Return L.

For the purposes of this operation, the 16-bit code units of the Strings are treated as code points in the Unicode
Basic Multilingual Plane. Surrogate code points are directly transferred from S to L without any
mapping.

The result must be derived according to the case mappings in the Unicode character database (this
explicitly includes not only the UnicodeData.txt file, but also the SpecialCasings.txt file that accompanies it in Unicode
2.1.8 and later).

NOTE 1 The case mapping of some characters may produce multiple characters. In this case the
result String may not be the same length as the source String. Because both toUpperCase and
toLowerCase have context-sensitive behaviour, the functions are not symmetrical. In other words,
s.toUpperCase().toLowerCase() is not necessarily equal to s.toLowerCase().

NOTE 2 The toLowerCase function is intentionally generic; it does not require
that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.

This function works exactly the same as toLowerCase except that its result is intended to yield the
correct result for the host environment’s current locale, rather than a locale-independent result. There will only
be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode
case mappings.

NOTE 1 The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.

NOTE 2 The toLocaleLowerCase function is intentionally generic; it does not
require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.

This function works exactly the same as toUpperCase except that its result is intended to yield the
correct result for the host environment’s current locale, rather than a locale-independent result. There will only
be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode
case mappings.

NOTE 1 The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.

NOTE 2 The toLocaleUpperCase function is intentionally generic; it does not
require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.

String instances inherit properties from the String prototype object and their [[Class]] internal property value is
"String". String instances also have a [[PrimitiveValue]] internal property, a length property,
and a set of enumerable properties with array index names.

The [[PrimitiveValue]] internal property is the String value represented by this String object. The array index named
properties correspond to the individual characters of the String value. A special [[GetOwnProperty]] internal method is used
to specify the number, values, and attributes of the array index named properties.

String objects use a variation of the [[GetOwnProperty]] internal method used for other native ECMAScript objects (8.12.1). This special internal method provides access to named properties corresponding to the
individual characters of String objects.

Assume S is a String object and P is a String.

When the [[GetOwnProperty]] internal method of S is called with property name P, the following
steps are taken:

Let desc be the result of calling the default [[GetOwnProperty]] internal method (8.12.1) on S with argument P.

Let resultStr be a String of length 1, containing one character from str, specifically the character
at position index, where the first (leftmost) character in str is considered to be at position 0, the
next one at position 1, and so on.

The Number prototype object is itself a Number object (its [[Class]] is "Number") whose value is +0.

The value of the [[Prototype]] internal property of the Number prototype object is the standard built-in Object prototype
object (15.2.4).

Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and the this
value passed to them must be either a Number value or an Object for which the value of the [[Class]] internal property is
"Number".

In the following descriptions of functions that are properties of the Number prototype object, the phrase “this
Number object” refers to either the object that is the this value for the invocation of the function or, if Type(this value) is Number, an object that is created as if by the expression new Number(this value) where Number is the standard
built-in constructor with that name. Also, the phrase “this Number value” refers to either the Number value
represented by this Number object, that is, the value of the [[PrimitiveValue]] internal property of this Number object or
the this value if its type is Number. A TypeError exception is thrown if the this value is neither an
object for which the value of the [[Class]] internal property is "Number" or a value whose type is Number.

The optional radix should be an integer value in the inclusive range 2 to 36. If radix not present or is
undefined the Number 10 is used as the value of radix.
If ToInteger(radix) is the Number 10 then this Number value is given as an argument to the ToString abstract operation; the resulting String value is returned.

If ToInteger(radix) is not an integer
between 2 and 36 inclusive throw a RangeError exception. If ToInteger(radix) is an integer from 2 to 36, but not 10, the result is a String
representation of this Number value using the specified radix. Letters a-z are used for digits
with values 10 through 35. The precise algorithm is implementation-dependent if the radix is not 10, however the algorithm
should be a generalisation of that specified in 9.8.1.

The toString function is not generic; it throws a TypeError exception if its this value is
not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

Produces a String value that represents this Number value formatted according to the conventions of the host
environment’s current locale. This function is implementation-dependent, and it is permissible, but not encouraged,
for it to return the same thing as toString.

NOTE The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything else.

The valueOf function is not generic; it throws a TypeError exception if its this value is
not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

Return a String containing this Number value represented in decimal fixed-point notation with fractionDigits
digits after the decimal point. If fractionDigits is undefined, 0 is assumed. Specifically, perform the
following steps:

Let n be an integer for which the exact mathematical value of n ÷ 10f –
x is as close to zero as possible. If there are two such n, pick the larger n.

If n = 0, let m be the String "0". Otherwise, let m be the String consisting
of the digits of the decimal representation of n (in order, with no leading zeroes).

If f ≠ 0, then

Let k be the number of characters in m.

If k ≤ f, then

Let z be the String consisting of f+1–k occurrences of the character
‘0’.

Let m be the concatenation of Strings z and m.

Let k = f + 1.

Let a be the first k–f characters of m, and let b be the remaining
f characters of m.

Let m be the concatenation of the three Strings a, ".", and b.

Return the concatenation of the Strings s and m.

The length property of the toFixed method is 1.

If the toFixed method is called with more than one argument, then the behaviour is undefined (see clause 15).

An implementation is permitted to extend the behaviour of toFixed for values of fractionDigits
less than 0 or greater than 20. In this case toFixed would not necessarily throw RangeError for such
values.

NOTE The output of toFixed may be more precise than toString for
some values because toString only prints enough significant digits to distinguish the number from adjacent number
values. For example,

Return a String containing this Number value represented in decimal exponential notation with one digit before the
significand's decimal point and fractionDigits digits after the significand's decimal point. If
fractionDigits is undefined, include as many significand digits as necessary to uniquely specify the
Number (just like in ToString except that in this case the Number is always output in exponential
notation). Specifically, perform the following steps:

If fractionDigits is not undefined and (f < 0 or f > 20), throw a
RangeError exception.

If x = 0, then

Let f = 0.

Let m be the String consisting of f+1 occurrences of the character ‘0’.

Let e = 0.

Else, x ≠ 0

If fractionDigits is not undefined, then

Let e and n be integers such that 10f ≤ n <
10f+1 and for which the exact mathematical value of n ×
10e–f – x is as close to zero as possible. If there are two such
sets of e and n, pick the e and n for which n ×
10e–f is larger.

Else, fractionDigits is undefined

Let e, n, and f be integers such that f ≥ 0, 10f ≤
n < 10f+1, the number value for n × 10e–f
is x, and f is as small as possible. Note that the decimal representation of n has
f+1 digits, n is not divisible by 10, and the least significant digit of n is not
necessarily uniquely determined by these criteria.

Let m be the String consisting of the digits of the decimal representation of n (in order, with no
leading zeroes).

If f ≠ 0, then

Let a be the first character of m, and let b be the remaining f characters of
m.

Let m be the concatenation of the three Strings a, ".", and b.

If e = 0, then

Let c = "+".

Let d = "0".

Else

If e > 0, then let c = "+".

Else, e ≤ 0

Let c = "-".

Let e = –e.

Let d be the String consisting of the digits of the decimal representation of e (in order, with no
leading zeroes).

Let m be the concatenation of the four Strings m, "e", c, and d.

Return the concatenation of the Strings s and m.

The length property of the toExponential method is 1.

If the toExponential method is called with more than one argument, then the behaviour is undefined (see clause 15).

An implementation is permitted to extend the behaviour of toExponential for values of
fractionDigits less than 0 or greater than 20. In this case toExponential would not necessarily
throw RangeError for such values.

NOTE For implementations that provide more accurate conversions than required by the rules
above, it is recommended that the following alternative version of step 9.b.i be used as a guideline:

Let e, n, and f be integers such that f ≥ 0, 10f ≤ n <
10f+1, the number value for n × 10e–f is x, and f is
as small as possible. If there are multiple possibilities for n, choose the value of n for which
n × 10e–f is closest in value to x. If there are two such
possible values of n, choose the one that is even.

Return a String containing this Number value represented either in decimal exponential notation with one digit before
the significand's decimal point and precision–1 digits
after the significand's decimal point or in decimal fixed notation with precision significant digits. If
precision is undefined, call ToString (9.8.1) instead.
Specifically, perform the following steps:

Let e and n be integers such that 10p–1 ≤ n <
10p and for which the exact mathematical value of n ×
10e–p+1 – x is as close to zero as possible. If there are two such
sets of e and n, pick the e and n for which n ×
10e–p+1 is larger.

Let m be the String consisting of the digits of the decimal representation of n (in order, with no
leading zeroes).

If e < –6 or e ≥ p, then

Let a be the first character of m, and let b be the remaining p–1
characters of m.

Let m be the concatenation of the three Strings a, ".", and b.

If e = 0, then

Let c = "+" and d = "0".

Else e ≠ 0,

If e > 0, then

Let c = "+".

Else e < 0,

Let c = "-".

Let e = –e.

Let d be the String consisting of the digits of the decimal representation of e (in order,
with no leading zeroes).

Let m be the concatenation of the five Strings s, m, "e", c, and
d.

If e = p–1, then return the concatenation of the Strings s and m.

If e ≥ 0, then

Let m be the concatenation of the first e+1 characters of m, the character ‘.’, and the remaining p– (e+1) characters of
m.

Else e < 0,

Let m be the concatenation of the String "0.", –(e+1) occurrences of the
character ‘0’, and the String m.

Return the concatenation of the Strings s and m.

The length property of the toPrecision method is 1.

If the toPrecision method is called with more than one argument, then the behaviour is undefined (see clause 15).

An implementation is permitted to extend the behaviour of toPrecision for values of precision
less than 1 or greater than 21. In this case toPrecision would not necessarily throw RangeError for
such values.

The Math object is a single object that has some named properties, some of which are functions.

The value of the [[Prototype]] internal property of the Math object is the standard built-in Object prototype object (15.2.4). The value of the [[Class]] internal property of the Math object is "Math".

The Math object does not have a [[Construct]] internal property; it is not possible to use the Math object as a constructor
with the new operator.

The Math object does not have a [[Call]] internal property; it is not possible to invoke the Math object as a function.

NOTE In this specification, the phrase “the Number value for x” has a
technical meaning defined in 8.5.

Each of the following Math object functions applies the ToNumber abstract operator to
each of its arguments (in left-to-right order if there is more than one) and then performs a computation on the resulting
Number value(s).

In the function descriptions below, the symbols NaN, −0, +0, −∞ and +∞ refer to the Number values
described in 8.5.

NOTE The behaviour of the functions acos, asin, atan,
atan2, cos, exp, log, pow, sin,
sqrt, and tan is not precisely specified here except to require specific results for certain
argument values that represent boundary cases of interest. For other argument values, these functions are intended to
compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of
approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library
for ECMAScript on a given hardware platform that is available to C programmers on that platform.

Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard)
that implementations use the approximation algorithms for IEEE 754 arithmetic contained in fdlibm, the freely
distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).

Returns an implementation-dependent approximation to the arc tangent of the quotient y/x of the arguments y and x, where the signs of y
and x are used to determine the quadrant of the result. Note that it is intentional and traditional for the
two-argument arc tangent function that the argument named y be first and the argument named x be
second. The result is expressed in radians and ranges from −π
to +π.

If either x or y is NaN, the result is NaN.

If y>0 and x is +0, the result is an implementation-dependent approximation to +π/2.

If y>0 and x is −0, the result is an implementation-dependent approximation to +π/2.

If y is +0 and x>0, the result is +0.

If y is +0 and x is +0, the result is +0.

If y is +0 and x is −0, the result is an implementation-dependent approximation to +π.

If y is +0 and x<0, the result is an implementation-dependent approximation to +π.

If y is −0 and x>0, the result is −0.

If y is −0 and x is +0, the result is −0.

If y is −0 and x is −0, the result is an implementation-dependent approximation to
−π.

If y is −0 and x<0, the result is an implementation-dependent approximation to
−π.

If y<0 and x is +0, the result is an implementation-dependent approximation to −π/2.

If y<0 and x is −0, the result is an implementation-dependent approximation to
−π/2.

If y>0 and y is finite and x is +∞, the result is +0.

If y>0 and y is finite and x is −∞, the result if an implementation-dependent
approximation to +π.

If y<0 and y is finite and x is +∞, the result is −0.

If y<0 and y is finite and x is −∞, the result is an implementation-dependent
approximation to −π.

If y is +∞ and x is finite, the result is an implementation-dependent approximation to
+π/2.

If y is −∞ and x is finite, the result is an implementation-dependent approximation to
−π/2.

If y is +∞ and x is +∞, the result is an implementation-dependent approximation to
+π/4.

If y is +∞ and x is −∞, the result is an implementation-dependent approximation to
+3π/4.

If y is −∞ and x is +∞, the result is an implementation-dependent approximation to
−π/4.

If y is −∞ and x is −∞, the result is an implementation-dependent
approximation to −3π/4.

Returns a Number value with positive sign, greater than or equal to 0 but less than 1, chosen randomly or pseudo
randomly with approximately uniform distribution over that range, using an implementation-dependent algorithm or strategy.
This function takes no arguments.

Returns the Number value that is closest to x and is equal to a mathematical integer. If two integer Number
values are equally close to x, then the result is the Number value that is closer to +∞. If x is already an integer, the result is x.

If x is NaN, the result is NaN.

If x is +0, the result is +0.

If x is −0, the result is −0.

If x is +∞, the result is +∞.

If x is −∞, the result is −∞.

If x is greater than 0 but less than 0.5, the result is +0.

If x is less than 0 but greater than or equal to -0.5, the result is −0.

NOTE 1Math.round(3.5) returns 4, but Math.round(–3.5) returns –3.

NOTE 2 The value of Math.round(x) is the same as the value of Math.floor(x+0.5), except when x is −0 or is less than 0 but greater than or equal to
-0.5; for these cases Math.round(x) returns −0, but Math.floor(x+0.5) returns
+0.

A Date object contains a Number indicating a particular instant in time to within a millisecond. Such a Number is
called a time value. A time value may also be NaN, indicating that the Date object does not represent a
specific instant of time.

Time is measured in ECMAScript in milliseconds since 01 January, 1970 UTC. In time values leap seconds are ignored. It
is assumed that there are exactly 86,400,000 milliseconds per day. ECMAScript Number values can represent all integers
from –9,007,199,254,740,992 to 9,007,199,254,740,992; this range suffices to measure times to millisecond precision
for any instant that is within approximately 285,616 years, either forward or backward, from 01 January, 1970 UTC.

The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly –100,000,000 days to
100,000,000 days measured relative to midnight at the beginning of 01 January, 1970 UTC. This gives a range of
8,640,000,000,000,000 milliseconds to either side of 01 January, 1970 UTC.

The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the value +0.

ECMAScript uses an extrapolated Gregorian system to map a day number to a year number and to determine the month and
date within that year. In this system, leap years are precisely those which are (divisible by 4) and ((not divisible by 100) or (divisible by
400)). The number of days in year number y is therefore
defined by

An implementation of ECMAScript is expected to determine the local time zone adjustment. The local time zone adjustment
is a value LocalTZA measured in milliseconds which when added to UTC represents the local standard time. Daylight
saving time is not reflected by LocalTZA. The value LocalTZA does not vary with time but depends only on the
geographic location.

An implementation of ECMAScript is expected to determine the daylight saving time algorithm. The algorithm to determine
the daylight saving time adjustment DaylightSavingTA(t), measured in milliseconds, must depend only on four
things:

The implementation of ECMAScript should not try to determine whether the exact time was subject to daylight saving
time, but just whether daylight saving time would have been in effect if the current daylight saving time algorithm had
been used at the time. This avoids complications such as taking into account the years that the locale observed daylight
saving time year round.

If the host environment provides functionality for determining daylight saving time, the implementation of ECMAScript
is free to map the year in question to an equivalent year (same leap-year-ness and same starting week day for the year)
for which the host environment provides daylight saving time information. The only restriction is that all equivalent
years should produce the same result.

Return an implementation-dependent choice of either ToInteger(time) or ToInteger(time) + (+0). (Adding a positive zero converts −0 to
+0.)

NOTE The point of step 3 is that an implementation is permitted a choice of internal
representations of time values, for example as a 64-bit signed integer or as a 64-bit floating-point value. Depending on
the implementation, this internal representation may or may not distinguish −0 and +0.

ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 Extended
Format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ

Where the fields are as follows:

YYYYis the decimal digits of the year 0000 to 9999 in the
Gregorian calendar.

-“-” (hyphen) appears literally twice in the string.

MMis the month of the year from 01 (January) to 12
(December).

DDis the day of the month from 01 to 31.

T“T” appears literally in the string, to indicate the beginning of the time element.

HHis the number of complete hours that have passed since
midnight as two decimal digits from 00 to 24.

:“:” (colon) appears literally twice in the string.

mmis the number of complete minutes since the start of the hour
as two decimal digits from 00 to 59.

ssis the number of complete seconds since the start of the
minute as two decimal digits from 00 to 59.

.“.” (dot) appears literally in the string.

sssis the number of complete milliseconds since the start of
the second as three decimal digits.

Zis the time zone offset specified as
“Z” (for UTC) or either
“+” or “-” followed by a time expressionHH:mm

This format includes date-only forms:

YYYYYYYY-MMYYYY-MM-DD

It also includes “date-time” forms that consist of one of the above date-only forms immediately followed by
one of the following time forms with an optional time zone offset appended:

THH:mmTHH:mm:ssTHH:mm:ss.sss

All numbers must be base 10. If the MM or
DD fields are absent “01” is used as the value. If the HH,
mm, or ss fields are absent “00” is used as the value and the value of
an absent sss field is “000”. The value of an absent time zone offset is
“Z”.

Illegal values (out-of-bounds as well as syntax errors) in a format string means that the format string is not a valid
instance of this format.

NOTE 1 As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to
distinguish the two midnights that can be associated with one date. This means that the following two notations refer to
exactly the same point in time: 1995-02-04T24:00 and 1995-02-05T00:00

NOTE 2 There exists no international standard that specifies abbreviations for civil time
zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this
reason, ISO 8601 and this format specifies numeric representations of date and time.

ECMAScript requires the ability to specify 6 digit years (extended
years); approximately 285,426 years, either forward or backward, from
01 January, 1970 UTC. To represent years before 0 or after 9999, ISO 8601 permits the expansion of the year representation, but only by
prior agreement between the sender and the receiver. In the simplified ECMAScript format such an expanded year
representation shall have 2 extra year digits and is always prefixed
with a + or – sign. The year 0 is considered positive and hence
prefixed with a + sign.

All of the arguments are optional; any arguments supplied are accepted but are completely ignored. A String is created
and returned as if by the expression (new Date()).toString() where Date is the standard built-in
constructor with that name and toString is the standard built-in method
Date.prototype.toString.

The parse function applies the ToString operator to its argument and interprets the
resulting String as a date and time; it returns a Number, the UTC time value corresponding to
the date and time. The String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending
on the contents of the String. The function first attempts to parse the format of the String according to the rules called
out in Date Time String Format (15.9.1.15). If the String does not conform to that format the
function may fall back to any implementation-specific heuristics or implementation-specific date formats. Unrecognisable
Strings or dates containing illegal element values in the format String shall cause Date.parse to return
NaN.

If x is any Date object whose milliseconds amount is zero within a particular implementation of ECMAScript,
then all of the following expressions should produce the same numeric value in that implementation, if all the properties
referenced have their initial values:

x.valueOf()

Date.parse(x.toString())

Date.parse(x.toUTCString())

Date.parse(x.toISOString())

However, the expression

Date.parse(x.toLocaleString())

is not required to produce the same Number value as the preceding three expressions and, in general, the value produced
by Date.parse is implementation-dependent when given any String value that does not conform to the Date Time
String Format (15.9.1.15) and that could not be produced in that implementation by the
toString or toUTCString method.

When the UTC function is called with fewer than two arguments, the behaviour is implementation-dependent.
When the UTC function is called with two to seven arguments, it computes the date from year,
month and (optionally) date, hours, minutes, seconds and
ms. The following steps are taken:

NOTE The UTC function differs from the Date constructor in two ways: it returns a time value
as a Number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.

The value of the [[Prototype]] internal property of the Date prototype object is the standard built-in Object prototype
object (15.2.4).

In following descriptions of functions that are properties of the Date prototype object, the phrase “this Date
object” refers to the object that is the this value for the invocation of the function. Unless explicitly noted
otherwise, none of these functions are generic; a TypeError exception is thrown if the this value is not an
object for which the value of the [[Class]] internal property is "Date". Also, the phrase “this time value” refers to the Number value for the time represented by this Date object, that is,
the value of the [[PrimitiveValue]] internal property of this Date object.

This function returns a String value. The contents of the String are implementation-dependent, but are intended to
represent the “date” portion of the Date in the current time zone in a convenient, human-readable form.

This function returns a String value. The contents of the String are implementation-dependent, but are intended to
represent the “time” portion of the Date in the current time zone in a convenient, human-readable form.